Opinion by Judge REINHARDT; Concurrence by Judge THOMAS; Dissent by Judge LEAVY.
REINHARDT, Circuit Judge:
This ease raises a number of important constitutional questions regarding the recently-enacted federal “three strikes” law, 18 U.S.C. § 3559(c) (1994).
Bryan K. Kaluna was convicted of bank robbery and conspiracy to commit bank robbery in violation of 18 U.S.C. § 2113(a) and 18 U.S.C. § 371.1 Because he had been convicted on one prior occasion of first-degree robbery and on several others of second-degree robbery, the district court sentenced him to life in prison under the three-strikes statute. Kaluna now appeals his sentence, arguing (1) that the statute’s recidivist sentencing scheme is facially unconstitutional; and (2) that, even if the sentencing law is constitutional in general, he has only two valid “strikes,” either because (a) the statute must be construed in a manner that excludes his other convictions or (b) the statutory burden-shifting provision used to establish his third strike is unconstitutional. We reject both his facial challenge to the three-strikes law generally and his statutory construction argument. We agree, however, that the statutory burden-shifting provision, [1072]*1072which requires defendants to prove by clear and convincing evidence that in the commission of certain prior offenses (i) a dangerous weapon was not used or threatened to be used and (ii) death or serious bodily injury did not occur, violates due process.
I
We first consider Kaluna’s general constitutional challenges to the “three strikes” law. The statute provides in relevant part that “[notwithstanding any other provision of law, a person who is convicted ... of a serious violent felony shall be sentenced to [mandatory] life imprisonment” if he has been convicted “on prior occasions ... of ... 2 or more serious violent felonies.” 18 U.S.C. § 3559(c)(1)..
Kaluna argues that the three-strikes statute violates five constitutional principles: (1) double jeopardy; (2) the separation of powers; (3) the Ex Post Facto Clause; (4) the prohibition against cruel and unusual punishment; and (5) the right to effective assistance of counsel. We have not had occasion to address these arguments with regard to the federal three-strikes statute, but three other circuits have rejected various challenges to the statute’s general constitutionality. See United States v. Rasco, 123 F.3d 222 (5th Cir.1997), cert. denied, — U.S. -, 118 S.Ct. 868, 139 L.Ed.2d 765 (1998); United States v. Washington, 109 F.3d 335, 337-38 (7th Cir.), cert. denied, — U.S. -, 118 S.Ct. 134, 139 L.Ed.2d 82 (1997); United States v. Farmer, 73 F.3d 836, 839-41 (8th Cir.), cert. denied, 518 U.S. 1028, 116 S.Ct. 2570, 135 L.Ed.2d 1086 (1996). We agree with these circuits, and with the district court, that the three-strikes statute’s recidivist sentencing scheme, harsh and inflexible as it may be, is facially constitutional in general.
First, Kaluna contends that the three-strikes statute violates the Double Jeopardy Clause because it imposes multiple punishment for the same offenses. Specifically, he argues that he has already served his punishment for his previous two “strikes,” and he cannot be punished again for them by counting them against him in the instant sentence. It is true that the “Double Jeopardy Clause protects against ... the actual imposition of two punishments for the same offense.” Witte v. United States, 515 U.S. 389, 115 S.Ct. 2199, 2204, 132 L.Ed.2d 351 (1995). But although the three-strikes statute might seem to violate this principle, the Supreme Court has long since determined that recidivist statutes do not violate double jeopardy because “the enhanced punishment imposed for the later offense ‘is not to be viewed as either a new jeopardy or additional penalty for the earlier crimes,’ but instead as ‘a stiffened penalty for the latest crime, which is considered to be an aggravated offense because a repetitive one.’” Id. 115 S.Ct. at 2206 (quoting Gryger v. Burke, 334 U.S. 728, 732, 68 S.Ct. 1256, 92 L.Ed. 1683 (1948)); see also Spencer v. Texas, 385 U.S. 554, 559-60, 87 S.Ct. 648, 17 L.Ed.2d 606 (1967); Moore v. Missouri, 159 U.S. 673, 16 S.Ct. 179, 40 L.Ed. 301 (1895). Thus, Kaluna’s argument cannot prevail.
Second, Kaluna contends that the three-strikes statute violates the fundamental constitutional principle of separation of powers because it impermissibly increases the discretionary power of prosecutors while stripping the judiciary of all discretion to eraft sentences. Alternately, Kaluna argues that this court should, in order to avoid constitutional difficulties, construe the statute to permit judges to apply the statute at their discretion. Again, we are compelled by precedent to reject both arguments. The Supreme Court has stated unequivocally that “Congress has the power to define criminal punishments without giving the courts any sentencing discretion.” Chapman v. United States, 500 U.S. 453, 467, 111 S.Ct. 1919, 114 L.Ed.2d 524 (1991); see also Mistretta v. United States, 488 U.S. 361, 364, 109 S.Ct. 647, 102 L.Ed.2d 714 (1989) (upholding the constitutionality of the federal sentencing guidelines in part because “the scope of judicial discretion with respect to a sentence is subject to congressional control”). Furthermore, the legislative history of the law leaves no doubt that Congress intended it to require mandatory sentences. See 103 Cong. Rec. S12,525 (daily ed. Aug. 25, 1994) (statement of Sen. Daschle) (stating that the three-strikes law “will require that Federal judges hand down mandatory life sentences”); Id. at S12,12544 (statement of Sen. Lautenberg) (emphasizing that under the three-strikes [1073]*1073law, three-time offenders are “put away for life_ And no ifs, ands or buts about it.”). The statute itself uses the words “mandatory” and “shall.” In any event, we cannot narrowly construe a law to avoid constitutional infirmity in this area because, given the principles previously announced by the Supreme Court, no constitutional question exists.
Third, Kaluna contends that the three-strikes statute violates the Ex Post Facto Clause because it changes the legal consequences of his prior bad acts. This contention also lacks merit. The Supreme Court and this court uniformly have held that recidivist statutes do not violate the Ex Post Facto Clause if they are “on the books at the time the [present] offense was committed.” United States v. Ahumada-Avalos, 875 F.2d 681, 683-84 (9th Cir.1989) (per curiam); see also Weaver v. Graham, 450 U.S. 24, 30, 101 S.Ct. 960, 67 L.Ed.2d 17 (1981). This statute was enacted in 1994 and Kaluna committed the instant offense in 1995.
Fourth, Kaluna argues that the three-strikes statute violates the proportionality guarantee of the Eighth Amendment. See U.S. Const, amend. VIII (“cruel and unusual punishments [shall not be] inflicted”). Supreme Court precedent once again forecloses his claim. In its most recent pronouncement on the subject, the Court held that “the eighth amendment ‘forbids only extreme sentences that are grossly disproportionate to the crime.’ ” United States v. Bland, 961 F.2d 123, 129 (9th Cir.1992) (quoting Harmelin v. Michigan, 501 U.S. 957, 1001, 111 S.Ct. 2680, 115 L.Ed.2d 836 (1991) (Kennedy, J., joined by O’Connor and Souter, JJ., concurring) (constituting the holding of the Court)). Moreover, while in evaluating the proportionality of a sentence, courts “must focus on the principal felony— the felony that triggers the life sentence,” Solem v. Helm, 463 U.S. 277, 290, 296 n. 21, 103 S.Ct. 3001, 77 L.Ed.2d 637 (1983), they must also recognize that legislatures may punish recidivists more severely than first-time offenders. Id. at 296, 103 S.Ct. 3001. Because the statute restricts its application to instances where both the defendants’ primary and past convictions are “serious violent felonies,”- the Court’s precedent makes it clear that Kaluna’s punishment for bank robbery is not sufficiently disproportionate to contravene the Eighth Amendment. See, e.g., Harmelin, 501 U.S. at 994-96, 111 S.Ct. 2680 (upholding life imprisonment for first offense of possessing 672 grams of cocaine); Rummel v. Estelle, 445 U.S. 263, 100 S.Ct. 1133, 63 L.Ed.2d 382 (1980) (upholding life sentence imposed after third nonviolent felony conviction).
Finally, Kaluna claims that he was deprived of the right to effective assistance of counsel because he was never advised that his guilty pleas to prior offenses could later constitute “strikes” leading to mandatory life imprisonment. Indeed, the three-strikes statute did not exist at the time of his guilty pleas to prior crimes. Following convention, however, we affirm, the district court’s determination to dismiss this claim without prejudice because it is properly raised through habeas corpus proceedings, not direct review. See Custis v. United States, 511 U.S. 485, 114 S.Ct. 1732, 1739, 128 L.Ed.2d 517 (1994) (ineffective assistance claims under recidivist statutes should be brought collaterally, not in sentencing procedures).
II
Haying held the three-strikes law generally constitutional, we reach Kaluna’s second contention. He argues that, even if the sentencing law is generally constitutional, he has only two valid “strikes” because either (1) the provision that affords the basis for his third strike must be construed in a manner that excludes his other convictions or (2) the statutory burden-shifting procedure used to establish his third strike is unconstitutional. We reject Kaluna’s first contention but agree with the second and, therefore, invalidate the relevant statutory provisions and vacate his sentence.
A
Provided the government follows the appropriate statutory procedures, courts are required under the three-strikes statute to impose life sentences on defendants who have committed three “serious violent felonies.” The statute defines serious- violent felonies, or “strikes,” under two different [1074]*1074schemes. First, it enumerates twelve specific crimes, such as murder, rape, and kidnapping, that automatically qualify as serious violent felonies. See 18 U.S.C. § 3559(c)(2)(F)(i). Second, it sets forth a group of crimes that essentially are presumed to be serious violent felonies — namely, robbery and arson, see id., and certain “un-enumerated” felonies that carry the same presumption. The latter category consists of:
any other offense punishable by a maximum term of imprisonment of 10 years or more that has as an element the use, attempted use, or threatened use of physical force against the person of another or that, by its nature, involves a substantial risk that physical force against the person of another may be used in the course of committing the offense.
18 U.S.C. § 3559(c)(2)(F)(ii). In the next subparagraph, however, the statute makes it clear that only certain acts of robbery and only certain unenumerated offenses are so “serious” or “violent” that they are to be treated as serious violent felonies. Section 3559(c)(3) provides that in order to “qualify” as a strike, a conviction for robbery or an unenumerated offense also must (i) involve the use, or threatened use, of a dangerous weapon or (ii) result in death or serious bodily injury. It also imposes on the defendant the burden of proving by clear and convincing evidence that these statutory conditions that qualify the robbery or other unenumerated offense for treatment as a serious violent felony were not met. The provision reads as follows:
(A) Robbery in certain cases. — Robbery, an attempt, conspiracy, or solicitation to commit robbery; or an offense described in paragraph (2)(F)(ii) shall not serve as a basis for sentencing under this subsection if the defendant establishes by clear and convincing evidence that—
(i) no firearm or other dangerous weapon was used in the offense and no threat of use of a firearm or other dangerous weapon was involved in the offense; and (ii) the offense did not result in death or serious bodily injury ... to any person.
18 U.S.C. § 3559(c)(3).2
At the time of sentencing, the district court considered whether six of Kaluna’s pri- or convictions counted as strikes: the instant offense under § 2113(a), a first-degree Hawaii state robbery conviction,3 and four second-degree Hawaii state robbery convictions. Kaluna raised little objection to terming the first two convictions strikes, but argued strenuously that, since under Hawaii law second-degree robbery does not require the use of a dangerous weapon,4 either none of his second-degree robbery convictions counted as strikes under the statute or, if they did, then the relevant statutory provisions were unconstitutional. He declined to offer any direct evidence regarding the facts or circumstances of any of his Robbery II convictions, choosing instead to rely solely on official court documents to argue that the offenses do not qualify under the statute and that the statutory burden-shifting provision is unconstitutional.
The district court rejected each of Kaluna’s arguments and ruled (1) that all six convictions qualified as strikes and (2) that this application of the statute was constitutional. It held specifically that the instant offense was strike one; that Kalun'a’s first-degree [1075]*1075robbery conviction was strike two; and that a second-degree robbery conviction for the robbery of Bill’s Bakery was strike three. Regarding the critical third strike, the court concluded that the elements in Hawaii’s Robbery II statute satisfied the category of “other offenses” described in § (2)(F)(ii)5 and that, “[e]qually significant,” Kaluna “failed to carry his burden under 18 U.S.C. § 3559(c)(3) to preclude the use of [the Bill’s Bakery robbery] as a basis for sentencing.” The court then noted that each of the other three second-degree robbery convictions also qualified as strikes, for the same reasons.
On appeal, Kaluna concedes that the instant robbery conviction and his prior conviction for first-degree robbery both constitute strikes. Both parties also agree that, since second-degree robbery “involves a substantial risk that physical force against the person of another may be used,” Kaluna’s “third” conviction (the Bill’s Bakery robbery)6 satisfies the three-strikes statute’s initial elements required for “other [qualifying] offense[s].” It is, in short, an unenu-merated presumptively serious violent felony. See § (2)(F)(ii). At this point, however, the parties part company. Kaluna first argues that his second-degree robbery conviction cannot satisfy the statute’s additional requirement that a dangerous weapon have been used or serious bodily injury have been caused because the provision must be construed “categorically.” In the alternative, he asserts that the statute’s burden-shifting provision violates due process. The government disagrees with both arguments.
B
We agree with the government that § (2)(F) and § (3)(A), read together, are clear on their face and cannot be given the construction Kaluna suggests. When the terms of a statute are unambiguous, we cannot alter their plain meaning. Demarest v. Manspeaker, 498 U.S. 184, 190, 111 S.Ct. 599, 112 L.Ed.2d 608 (1991); United States v. Valencia-Andrade, 72 F.3d 770, 774 (9th Cir.1995). In this ease, if Congress had not explicitly required a determination that no dangerous weapon “was used in the offense” or that “the offense did not result” in serious bodily injury, § (3)(A), we might be able to entertain Kaluna’s suggestion that we adopt a “categorical” approach to the qualifying offenses. See Taylor v. United States, 495 U.S. 575, 110 S.Ct. 2143, 109 L.Ed.2d 607 (1990) (discussing categorical approach). Given the statutory language, however, we are not free to do so. The statute, in explicit terms, requires courts to look not to the elements of the statute under which a defendant was convicted, but rather to the specific facts underlying the particular conviction. In short, the words of the three-strikes statute make it clear that Congress intended courts to employ a fact-specific, ease-by-case approach when determining whether prior convictions for robbery and the unenumerat-ed offenses constitute serious violent felonies, or “strikes.”
C
Since the relevant provisions of the statute are susceptible to only one possible construction — namely, that courts must employ a fact-specific approach as to whether particular convictions constitute strikes — we are compelled to decide whether the burden-shifting provision comports with due process. Kaluna argues that the provision is unconstitutional because it requires him to prove that the facts surrounding his prior conviction do not satisfy one of the statute’s prerequisites for a strike and because its “clear and convincing evidence” standard is, in any event, too high. Thus, the specific issue we must [1076]*1076decide is whether, once Congress has determined that certain offenses constitute “serious violent felonies” and therefore “qualify” as strikes (leading to a sentence enhancement) only if a dangerous weapon was used or serious bodily injury resulted, it may shift the burden to the defendant to disprove that qualifying component of the statute, and, if so, whether it may require him to do so by means of clear and convincing evidence.7
This dispute presents an issue of first impression in this circuit. In United States v. Morrison, 113 F.3d 1020 (9th Cir.), cert. denied, - U.S. -, 118 S.Ct. 583, 139 L.Ed.2d 421 (1997), we held that the defendant was barred from arguing that the statute’s burden-shifting procedure was unconstitutional because he was challenging a “strike” based on a guilty plea to both robbery and a statute prohibiting firearm use. We explained that, under such circumstances, the provision’s “burden of proof [was] ... irrelevant” because, under res judi-cata principles, the defendant did “not have the right to rebut facts necessarily adjudicated by the earlier judgment,” id. át 1021-22. We found it unnecessary, therefore, to reach the question whether the statute violated due process by imposing its burden-shifting provision as the means of establishing the “dangerous weapon” element of a robbery-strike. Id. at 1021. This case squarely presents that issue. In none of the criminal judgments encompassing Kaluna’s second-degree robbery convictions (the convictions, and particularly Bill’s Bakery, that would constitute his third strike), was Kaluna convicted of an offense that necessarily included the use, or threatened use, of a dangerous weapon or necessarily resulted in death or serious bodily injury; thus, the district court was required -to invoke the statute’s fact-finding procedure — its burden-shifting mechanism-in order to establish his third strike.8
Our inquiry into the provision’s constitutionality consists of two steps. First, we consider whether § (3)(A)’s “dangerous weapon” or “serious bodily injury” component constitutes an “essential statutory ingredient” or whether it is an “exception” to the statute’s reach — and thus an “affirmative [1077]*1077defense.” We conclude that it is an essential ingredient — -in other words, a statutory element. Second, we consider whether the burden of proof imposed by the three-strikes statute regarding this element comports with due process. We hold that it does not.
Whether the provision at issue prescribes an element or an exception is of critical importance. The government ordinarily must carry the burden of proof regarding elements, or facts that are “essential” to a criminal statute’s application. See, e.g., Almendarez-Torres v. United States, — U.S. -,-, 118 S.Ct. 1219, 1228, 140 L.Ed.2d 350 (1998); McMillan v. Pennsylvania, 477 U.S. 79, 85, 106 S.Ct. 2411, 91 L.Ed.2d 67 (1986); Patterson v. New York, 432 U.S. 197, 215, 97 S.Ct. 2319, 53 L.Ed.2d 281 (1977); Mullaney v. Wilbur, 421 U.S. 684, 692-96, 95 S.Ct. 1881, 44 L.Ed.2d 508 (1975). On the other hand, it is a well established rule of statutory construction that “a defendant who relies upon an exception to a statute ..., whether in the same section of the statute or elsewhere, has the burden of establishing and showing that he comes within that exception.” United States v. Freter, 31 F.3d 783, 788 (9th Cir.) (internal quotation marks omitted); accord United States v. Gravenmeir, 121 F.3d 526, 528 (9th Cir.1997); Walker v. Endell, 850 F.2d 470, 472-73 (9th Cir.1987).
While the question of essential fact versus exception may arise regarding either a statute establishing a criminal offense or a statute prescribing a criminal sentence, the manner in which courts decide whether a clause constitutes an essential fact or an affirmative defense is the same for both kinds of criminal statutes. See McMillan, 477 U.S. at 85, 91, 106 S.Ct. 2411 (stating that sentencing laws are “differen[t] in degree” than statutes establishing offenses, but indicating that the same general principles “eontrol[ ]”). As the Sixth Circuit has explained, “[t]he thrust of an inquiry under Mullaney and Patterson is whether the [legislature] has defined the elements of the crime so as to presume a fact essential to guilt and then compelled the accused to negate that element of the crime.” Carter v. Jago, 637 F.2d 449, 455 (6th Cir.1980).9
We have never established a specific test for deciding whether a particular clause must be considered an element or an affirmative defense, but we have consistently lopked to three factors: (1) the form of the statute, particularly whether the clause at issue appears in a separate provision; ' (2) the breadth of the alleged “exception” in relation to the overall provision; and (3) the ease with which the respective parties can present evidence to prove the matter, especially whether one party might be forced to prove a negative. See Gravenmeir, 121 F.3d at 528; Freter, 31 F.3d at 788; United States v. Hester, 719 F.2d 1041, 1042 (9th Cir.1983); see also United States v. Durrani, 835 F.2d 410, 420-21 (2d Cir.1987) (adopting test that considers essentially the same factors). The Supreme Court directs us also to consider a fourth factor when the historical facts permit such an analysis: whether the clause involves a matter that is traditionally an element of a crime (or sentencing enhancement) or traditionally constitutes an affirmative defense. See, e.g., Mullaney, 421 U.S. at 692-96, 95 S.Ct. 1881 (invalidating statute that shifted burden of proof regarding malice in murder statute largely because malice is traditionally an element of murder); Leland v. Oregon, 343 U.S. 790, 796-97, 72 S.Ct. 1002, 96 L.Ed. 1302 (1952) (holding that states may require a defendant to prove insanity beyond a reasonable doubt primarily because insanity has always been an affirmative defense). While [1078]*1078in our four earlier eases there was no relevant history that permitted application of the fourth factor, that factor is of considerable importance here.10
A careful analysis of § (2)(F) and § (3)(A) reveals that the latter’s “dangerous weapon” or “death or serious bodily injury” provision represents an additional “element” defining whether robberies or unenumerated offenses qualify as serious violent felonies, and does not merely constitute an exception or affirmative defense. First, while it is true that the “dangerous weapon”/ “serious bodily injury” requirement is in a separate provision from the definition of robbery and unenu-merated offenses, this fact is not talismanic. See, e.g., Patterson, 432 U.S. at 210, 97 S.Ct. 2319; Freter, 31 F.3d at 788. If it were, Congress could always shift the burden of proof regarding elements of a crime simply by placing them in separate provisions and calling them “exceptions.” A legislature cannot, for example, define murder as the unlawful killing of another person and then shift the burden of proof to the defendant to establish that he did not act with malice aforethought. See Mullaney, 421 U.S. at 703-04, 95 S.Ct. 1881 (striking down such a law). We, therefore, must consider the other three factors to determine whether Congress enacted a legitimate exception or a thinly disguised element — in other words, whether the statute requires the court to engage in unlawful burden-shifting.
Each of the other three factors strongly supports the conclusion that § (3)(A) sets forth an actual element of the statute. First, crimes in which (i) a dangerous weapon is used or threatened or (ii) death or serious bodily injury results constitute a sizeable percentage of crimes that have as an element the use of force, the threatened use of force, or a substantial risk of the use of force. We have previously held that statutory provisions constituted exceptions only when they excluded from the statute’s reach comparatively narrow ranges of conduct. See Gravenmeir, 121 F.3d at 528 (holding that a “lawful transfer or lawful possession of a machinegun that was lawfully possessed before” May 19, 1986 is an exception to the crime of transferring or possessing a machinegun); Freter, 31 F.3d at 787-88 (holding that a federally permitted release is an exception to the crime of failing to notify the appropriate agency of hazardous substance releases); Hester, 719 F.2d at 1042 (holding that Indian status is an exception to statute allowing prosecutions of non-Indians for crimes committed in Indian country against Indians). Other circuits are in accord. See, e.g., Durrani, 835 F.2d at 421 (holding a provision an affirmative defense in part because the statutory prohibition was broad and the exception was narrow); United States v. Chodor, 479 F.2d 661, 663-64 & n. 2 (1st Cir.1973) (same). The breadth of the conduct covered by § (3)(A) stands in stark contrast to the strictly limited, discrete slices of conduct we have previously recognized as exceptions.
Next, reading § (3)(A) as an exception would require the defendant to prove the negative (by clear and convincing evidence) with respect to the use or threatened use of a gun or the occurrence of death or serious bodily injury. Proving the absence of these circumstances, like proving the absence of any set of facts, is exceedingly difficult. Courts have been particularly hesitant to require defendants to prove exculpatory facts that are beyond their unique ability to establish. Indeed, we have condoned such a requirement only when the defendant had some particular ability to prove the necessary facts.11 In Freter, for example, when we held that the defendant could be required to prove that he owned a permit allowing hazardous substance releases, we explained that “[i]t is far easier for the defendant to present evidence that the release is federally [1079]*1079permitted under one of these statutes than for the government to produce evidence that the release is not permitted under any of them.” 31 F.3d at 788; see also Gravenmeir, 121 F.3d at 528 (emphasizing fact that it is “easier” for defendants to show whether they obtained machinegun before 1986); Hester, 719 F.2d at 1043 (“It is far more manageable for the defendant to shoulder the burden of producing evidence that he is a member of a federally recognized tribe than it is for the Government to produce evidence that he is not a member of any one of the hundreds of such tribes.”). Likewise, defendants have traditionally been required to prove insanity claims because knowledge of their mental states is uniquely within their control. See Leland, 343 U.S. at 796-97, 72 S.Ct. 1002. It is far easier, however, for the government to prove the use or presence of a dangerous weapon or that death or serious bodily injury resulted than it is for the defendant to demonstrate the opposite, particularly in connection with crimes that may have occurred a number of years ago. This is in large part because the government, with its ready access to official files and records, and its superior ability to locate or track down individuals, is in a better position to obtain the testimony of victims of, or percipient-witnesses to, past offenses. Furthermore, the facts at issue are the type that the government regularly endeavors to prove in ordinary criminal eases where it customarily is required to carry its burden of showing that specific events happened.
Finally, we find it highly significant that the use of a firearm or dangerous weapon is traditionally an element of an offense or of a sentencing enhancement — an element that must be proved by the government. See, e.g., 18 U.S.C. § 111(b) (enhancing penalty for assault against federal officers when government establishes use of a dangerous weapon or serious bodily injury); 18 U.S.C. § 924(c) (criminalizing the use of firearm in relation to a drug offense); United States Sentencing Commission, Guidelines Manual § 2Dl.l(b)(l) (1997) (providing sentence enhancement for possession of a dangerous weapon during certain drug offenses); McMillan, 477 U.S. at 91, 106 S.Ct. 2411 (holding that state must prove use of a firearm in sentence enhancement statute). In fact, the very federal statutes referred to in the three-strikes law, which define the elements of robbery, provide for a separate— and more serious — crime and sentence if the government proves that the defendant used a dangerous weapon. See 18 U.S.C. §§ 2113(d) & 2118(c)(1).12 And, as Hawaii’s statutes demonstrate, armed robbery is traditionally a different and far more serious state crime than simple robbery. Compare supra n. 3 (defining first-degree robbery) with n. 4 (defining second-degree robbery); see also United States v. Mancusi, 448 F.2d 233, 235 (2d Cir.1971) (emphasizing same distinction in New York law). In Hawaii, as in other states, the more serious (and more violent) crime is proven when the prosecutor carries the burden of establishing that a dangerous weapon was used. Thus, it is difficult to escape the conclusion that § (3)(A) shifts the burden of proof to the defendant to prove something that, prior to the three-strikes law, the government was traditionally required to prove for purposes of both conviction and sentence enhancement.
The only exception to the practice of expressly defining the use of a dangerous weapon as an element of a statute appears in a provision known as the “safety valve” law, which was passed in the same act as the three-strikes law,13 United States Sentencing [1080]*1080Guideline § 5C1.2,18 U.S.C. § 3553(f). That provision allows defendants convicted of certain nonviolent drug offenses to receive a downward departure from the mandatory minimum if they prove by a preponderance of the evidence that they meet five conditions, one being that they did not use a dangerous weapon in the commission of the offense. See United States v. Ajugwo, 82 F.3d 925, 929 (9th Cir.1996), cert. denied, — U.S. -, 117 S.Ct. 742, 136 L.Ed.2d 680 (1997).14 The safety-valve provision, however, is easily distinguished, from the mandatory lifetime provision of the three-strikes statute. The former, in contrast to the latter, applies only to events that occurred during the offense for which the defendant is currently being tried. All of the witnesses and evidence are already present at the proceedings, making it far easier for the defendant to prove what happened. More important, the safety valve provision permits a reduction in the sentence ordinarily imposed for the offense, while the “dangerous weapon” requirement in the three-strikes statute mandates a significant enhancement — a substantial increase in the punishment. In other words, Congress viewed the safety-valve requirements of § 5C1.2 as “mitigating” factors, see Ajugwo, 82 F.3d at 926; the reduced sentence is relief that the defendant may obtain by proving that he is entitled to receive it — by establishing, inter alia, that he did not use a dangerous weapon. If the defendant fails to carry his burden, he simply receives the normal sentence for the offense he committed. By contrast, under the three-strikes statute, if a defendant fails to meet § (3)(A)’s burden regarding the use of a dangerous weapon, an additional penalty is imposed — a greatly enhanced sentence. It is only the three-strikes statute, therefore, that “presume[s] ... [the] essential fact,” McMillan, 477 U.S. at 83, 106 S.Ct. 2411 (quotation marks omitted and emphasis added), of the use of a dangerous weapon and then bases increased punishment on that presumption.
In sum, we conclude that § (3)(A) sets forth an element, or essential ingredient, of the three-strikes law’s enhancement provision — an element that qualifies certain robberies and other unenumerated offenses as serious violent felonies, or strikes. Under the statute, only armed robberies and other armed offenses, or threatened armed robberies or other armed offenses, or robberies and other offenses' that result in death dr serious bodily injury, constitute strikes that súbject the defendant to life imprisonment. Yet the statute requires the defendant to prove that he was not armed and did not cause any such injury; that he did not commit the type of crime that the three-strikes law encompasses; that the offense he committed does not qualify as a strike. There can be no doubt that proof of (1) the use or threat to use a dangerous weapon or (2) the occurrence of death or serious bodily injury is proof of an element necessary to the classification of the offense as a strike. What the three-strikes statute does, therefore, is to presume the existence of the element and then require the defendant to disprove its existence by clear and convincing evidence.
We now turn to the second part of our inquiry: whether the burden of proof allocated by Congress regarding the dangerous weapon/death or serious injury element comports with due process.
For starters, it is apparent that if the provisions of the three-strikes statute that are at issue here applied to the guilt phase of a prosecution, they would be unconstitutional. The Supreme Court made clear in Patterson and Mullaney that a statute proscribing criminal conduct violates due process whenever it “shift[s] the burden of proof to the defendant by presuming [an] ingredient [of an offense] upon proof of the other elements of the offense.” Patterson, 432 U.S. at 215, 97 „ S.Ct. 2319 (holding that statute requiring defendant to prove that he suffered from extremé emotional disturbance did not run afoul of this rule because that was an [1081]*1081affirmative defense to offense); see also Mullaney, 421 U.S. at 692-704, 95 S.Ct. 1881 (holding that statute requiring defendant to prove that he acted in heat of passion to avoid murder conviction violated due process because malice aforethought is element of murder); cf. In re Winship, 397 U.S. 358, 364, 90 S.Ct. 1068, 25 L.Ed.2d 368 (1970) (holding that “the Due Process Clause protects the accused against conviction except upon proof beyond a reasonable doubt of every fact necessary to constitute the crime with which he is charged”). The burden regarding all such “ingredients” must rest with the prosecution.
While neither the Supreme Court nor this court has ever been confronted directly with the question of the constitutionality of a sentence enhancement statute that shifts the burden of proof of an “essential ingredient” to the defendant, the Court’s recent sentencing decisions provide considerable guidance in this regard. Those decisions strongly suggest that the three-strikes law’s burden-shifting scheme must be held unconstitutional.
It has long been settled that due process protections apply generally to recidivist sentencing statutes. See Specht v. Patterson, 386 U.S. 605, 610, 87 S.Ct. 1209, 18 L.Ed.2d 326 (1967). Furthermore, the Court recently has warned, with respect to sentencing enhancements based on prior offenses, that even when the government has the burden of proof “the practical difficulties and potential unfairness of a factual approach are daunting.” Taylor, 495 U.S. at 601, 110 S.Ct. 2143. The Court has never had occasion, however, to prescribe formally any minimum burden of proof that applies to essential ingredients of qualifying offenses in recidivist sentencing enhancement statutes. See Almendarez-Torres, 118 S.Ct. at 1223.
The Court nevertheless recently reaffirmed the rule that “application of the preponderance of the evidence standard at sentencing generally satisfies due process.” United States v. Watts, 519 U.S. 148, 117 S.Ct. 633, 637, 136 L.Ed.2d 554 (1997) (per curiam); see also McMillan, 477 U.S. at 91-92, 106 S.Ct. 2411.15 We, along with other circuits, have understood the Court’s opinions to say that while that standard provides an acceptable minimum in most eases, in certain instances the government may have to meet a standard that is even higher. Citing McMillan, 477 U.S. at 88, 106 S.Ct. 2411, we announced: “[W]hen a sentencing factor has an extremely disproportionate effect on the sentence relative to the offense of conviction,” the government may have to meet a “clear and convincing evidence” standard. United States v. Restrepo, 946 F.2d 654, 659 (9th Cir.1991) (en banc); see also United States v. Lombard, 72 F.3d 170, 186-87 (1st Cir.1995) (holding that where sentence enhancement is “enormous” downward departure was allowed because enhancing a sentence in such a manner based on a preponderance of the evidence may have “exceeded” constitutional limits); United States v. Gigante, 39 F.3d 42, 48 (2d Cir.1994) (stating that preponderance of the evidence is a threshold basis for sentencing but that courts may require higher standards to be met where the factor leads to a “substantially enhanced sentencing range”); United States v. Townley, 929 F.2d 365, 369 (8th Cir.1991) (stating that “the preponderance standard the [McMillan ] Court approved in the garden variety sentencing determinations may fail to comport with due process where, as here, a sentencing enhancement factor becomes ‘a tail which wags the dog of the substantive offense’”) (quoting McMillan, 477 U.S. at 88, 106 S.Ct. 2411); United States v. Kikumura, 918 F.2d 1084, 1101-02 (3d Cir.1990) (holding that a factor that enormously increased defendant’s sentence had to be proven by clear and convincing evidence).
Given this case law, it is not surprising that neither the Supreme Court nor any circuit court has ever suggested that Con[1082]*1082gress may shift the burden of proof regarding an essential factor in any kind of sentencing enhancement; let alone one as extreme as the law before us.16 The Court’s reasoning in Mullaney, in fact, suggested, in emphatic terms, that such burden-shifting would be unconstitutional. The Court said that under a statute that shifts the burden of proof regarding an essential fact, a defendant “can be given a life sentence when the evidence indicates that it is as likely as not that he deserves a significantly lesser sentence. This is an intolerable result_” 421 U.S. at 703, 95 S.Ct. 1881. While that statement was made with reference to a statute proscribing criminal conduct, it describes precisely the result that would occur were we to uphold the burden-shifting provision of the three-strikes statute. Moreover, the Court in Watts, consistent with the Mullaney analysis, held that whether or not a defendant has been acquitted of using or carrying a firearm during the offense of conviction, his sentence may be enhanced for possessing a firearm during that offense “so long as that conduct has been proved by a preponderance of the evidence.” Id. 117 S.Ct. at 638 (emphasis added). This statement forcefully supports the conclusion that the dangerous weapon ingredient in this case must also be proved by at least a preponderance of the evidence.
Indeed, the three-strikes enhancement has both of the hallmarks of the type of sentence enhancement that we, and other circuits, have stated may require the government to prove the elements by more than a preponderance of the evidence. First, the enhancement is “enormous.” Lombard, 72 F.3d at 186. Without the three-strikes law, Kaluna faced 188r235 months, or from just over 15 to just under 20 years, in prison. At the time of sentencing he was 33 years old. The practical effect of a life sentence could well be that Kaluna would be imprisoned in the neighborhood of 35 to 45 years. In other words, application of the three-strikes provision could easily double or triple Kaluna’s already harsh sentence. By any measure, therefore, “the consequences of an erroneous determination [of the facts] are dire.” Cooper v. Oklahoma, 517 U.S. 348, 116 S.Ct. 1373, 1381, 134 L.Ed.2d 498 (1996). Second, unlike the law in McMillan, 477 U.S. at 81-82, 106 S.Ct. 2411, the three-strikes enhancement allows — indeed, requires (when the triggering offense is not punishable by life in prison) — the court to impose a sentence above the statutory maximum — in fact, far above: life in prison.
We therefore conclude that due process does not allow courts to presume, simply because a defendant has been convicted of a crime such as robbery that “involves a substantial risk that physical force ... may be used,” § (2)(F)(ii), that he either actually used or threatened to use a dangerous weapon or actually caused a death or serious bodily injury, § (3)(A), 'and then, on the basis of that presumption, to enhance his sentence from a term of years to life imprisonment. It is one thing to enhance a defendant’s sentence by a few years because the government has shown by at least a preponderance of the evidence that, for example, he used a firearm in an offense; it’s quite another to enhance a defendant’s sentence to life in prison because did not establish that he did not use a firearm. Indeed, in this case the vice is particularly severe: the defendant must carry his burden “by clear and convincing evidence.” § (3)(A). The three-strikes law thus forces the defendant to bear the unacceptable risk that he will be sentenced to life in prison even if he “has already demonstrated that he ... more likely than not” did not use a gun in a “qualifying” offense. Cooper, 116 S.Ct. at 1381 (holding that statute violated due process because it presumed defendants competent to stand trial unless they proved otherwise by clear and convincing evidence). Such a consequence is, as the Court has already said, “intolerable.” Mullaney, 421 U.S. at 703, 95 S.Ct. 1881.
[1083]*1083We hold that the parts of the statute that relate to whether prior robbery convictions and “other offense[s]” qualify as strikes— namely, §§ (3)(A), (2)(F)(ii), and the robbery clause of § (2)(F)(i) — -are unconstitutional. We must invalidate these provisions because each is inextricably intertwined with the other. See Hill v. Wallace, 259 U.S. 44, 70-71, 42 S.Ct. 453, 66 L.Ed. 822 (1922) (requiring courts to invalidate unconstitutional regulations that are “so interwoven ... that they cannot be separated”). If we were to invalidate only the actual burden-shifting provision, and leave the presumptive classification provisions in effect, our ruling would produce an effect precisely the opposite of that which Congress intended in adopting § (3)(A): we would be enlarging the number of robbery convictions and “other offense[s]” that would qualify as strikes, while the whole purpose of § (3)(A) is to reduce the number of such offenses that qualify as strikes by limiting them to those that are most serious — offenses that involve acts or consequences not present in the case of simple robberies. There is no indication that Congress would have wished to classify all “robberies” and “other offense[s]” (as described in § (2)(F)(ii)) as strikes. Its actions show that its intent was directly contrary.17
Ill
Since we conclude that the three-strikes statute is constitutional in general, but that the provisions governing certain offenses that qualify only under specified conditions are unconstitutional, we must confront the question of severability. See, e.g., Alaska Airlines v. Brock, 480 U.S. 678, 684, 107 S.Ct. 1476, 94 L.Ed.2d 661 (1987). “[W]henever an act of Congress contains unobjectionable provisions separable from those found to be unconstitutional, it is the duty of this court to so declare, and to maintain the act in so far as it is valid.” El Paso & Northeastern R.R. v. Gutierrez, 215 U.S. 87, 96, 30 S.Ct. 21, 54 L.Ed. 106 (1909). In determining whether unconstitutional provisions are severable, we engage in a two-part inquiry. Board of Natural Resources v. Brown, 992 F.2d 937, 948 (9th Cir.1993). “First, we inquire whether the Act which remains after the unconstitutional provisions are excised is ‘fully operative,’ ” id., or “functionally independent” of the rest. Alaska Airlines, 480 U.S. at 684, 107 S.Ct. 1476. Second, “we then inquire whether Congress would have enacted the constitutional provisions of the Act independently of the unconstitutional provisions.” Brown, 992 F.2d at 948.
We conclude that the three-strikes law is fully operative without the unconstitutional clauses: §§ (3)(A), (2)(F)(ii), and the robbery clause of § (2)(F)(i). The bulk of the statute remains intact, and is functionally independent. Convictions on any one of twelve or thirteen of the original fourteen enumerated felonies still count as strikes;18 and three such convictions still require imposition of a mandatory life sentence upon an appropriate request by the prosecutor. That some offenses have been struck has no effect on those that remain.
Equally important, we can be “confident” that Congress would have enacted the law independently of the offending provisions. See Brown, 992 F.2d at 948. The remaining felonies are those which are so serious that Congress decided to enhance the punishment of defendants with three or more such convictions regardless of whether the additional requirements mandated under § (3)(A) were satisfied. Since Congress was willing to classify twelve specific crimes as “serious violent felonies” unconditionally and to make several [1084]*1084others strikes only if the defendant used a dangerous weapon or caused serious bodily injury, we can safely assume it would have enacted the law with only the twelve specific crimes, if necessary. Put in other terms, without the combination of the clauses that we strike from the statute, the sentence enhancement statute will still “function in a manner consistent with the intent of Congress,” Alaska Airlines, 480 U.S. at 685, 107 S.Ct. 1476.
Kaluna’s sentence is VACATED and REMANDED for further proceedings consistent with this opinion.