Every year, a sitting Iranian president, whether Hassan Rouhani or Mahmoud Ahmadinejad, instructs the UN General Assembly that Israel represents some sort of defiling historical error, a mistake that should somehow be “rectified.” On occasion, Iran’s president, plainly, and with obviously full authority from (Supreme Leader) Grand Ayatollah Ali Khamenei, goes beyond such narrowly focused denunciation, and offers an alleged rationale for Israel’s “disappearance.” What has yet to be examined, in any serious fashion, is whether these Iranian presidents have actually been urging genocide, and whether, in an aptly defensive response, the Israeli prime minister still retains proper legal authority to strike first.At a minimum, however, Israel has every right to request a General Assembly resolution calling for Iran’s formal expulsion from the United Nations. While such a diplomatic and jurisprudential rejoinder to Iran’s presumptively genocidal pleas could be permissible, it would also likely represent, at best, only a preliminary first step toward improving Israel’s national security.Under international law, genocide has a very precise meaning. This specific content is authoritatively defined at the Convention on the Prevention and Punishment of the Crime of Genocide. According to this 1948 treaty, which entered into force in 1951, and is also binding upon non-signatory states as customary international law, pertinent violations are not confined to any specific enumerated acts “committed with “intent to destroy.” They also include “conspiracy to commit genocide,” and “incitement to commit genocide.”Today, in what amounts to a conveniently mobilizing mantra, Rouhani hints at winning a divinely-mandated war against the “Zionist regime.” Significantly, in law, war and genocide are not mutually exclusive. Genocide, unlike certain earlier Nuremberg-defined crimes against humanity, is not linked to belligerency. It can be carried out “in time of peace, or in time of war.”No state is ever obliged to passively await genocide.This principle, “peremptory” because it is so fundamental and overriding, includes those exterminatory belligerencies that masquerade as war. Under both codified and customary legal rules, every state maintains an “inherent right” to individual or collective self-defense.In express violation of the 1948 Genocide Convention and its derivative norms, Iranian calls for Israel’s “disappearance” are more than sinister cartographic fantasies. They are also determinably genocidal provocations.Further, in view of its corollary unwillingness to abide by obligations under both the UN Charter, and the 1968 Nuclear Nonproliferation Treaty (NPT), Iran also willfully disregards the binding norms of general international human rights law.Unhindered, Iran is now finalizing its construction of a nuclear weapons capability. The Tehran regime regards nuclear weapons as an acceptable means to create “a world without Zionism.” As for any sort of reconciliation with Israel, Iran’s former president triumphantly declared: “Anybody who recognizes Israel will burn in the fire of the Islamic nation’s fury.”This declaration is not especially hard to decipher.Israel likely faces a zero-sum “game” with Iran, a life-or-death contest in which one state’s ultimate victory will require the other’s total defeat. Very soon, therefore, Israel’s leaders could have to make unprecedented final decisions on launching defensive first strikes.Operational and jurisprudential judgments on this urgent issue are altogether discrete, and thus need to be appraised separately. Here, we are interested only in the second standard of assessment. Could such strikes be legal? Would the case for Israeli legality be strengthened by Iran’s willingness to go beyond aggression to genocide? And does the Genocide Convention address itself anywhere to the vital security issue of anticipatory self-defense? On June 7, 1981, Israel launched Operation Opera against Saddam Hussein’s then-developing nuclear reactor outside of Baghdad. Officially, this preemptive attack on Osirak was an expression of anticipatory self-defense. Interestingly, however, because Iraq had always considered itself to be formally at war with Israel, the Jewish state could just as correctly have regarded this essential act of protection as something else.Back in 1981, taking an alternative legal position, prime minister Menachem Begin could have justified Operation Opera as a permissible tactical action in the wider context of a longstanding and still-ongoing belligerency. Had he done so, Israel could then have pointed out that both of the pertinent legal obligations applying here had been fully satisfied. These obligations are the always twin obligations of “just cause” (here, warding off an existential threat), and “just means” (minimizing collateral harms).Legally, Begin chose to link Operation Opera to preventing “another Holocaust.” Historically, the core rationale of including anticipatory self-defense under customary international law had been the prevention of aggression, not genocide. After the Holocaust, it became plain that the prerogatives of sovereignty in world law could no longer remain absolute, and that the once-legitimate cover of “domestic jurisdiction” would now have to exclude certain egregious crimes against human rights.In essence, the international legal order now accepts and expects that all states will feel co-responsible for each other, including the prevention of genocide.Until now, there have been no recognized examples of anticipatory self-defense as a specifically preventative anti-genocide measure under international law.The interminable Iraq and Afghanistan wars provide an example of an American preemption strategy for national self-defense against terrorism, but not against an expected genocide. From the standpoint of permissibility under international law, even this restricted example of preemption is exceedingly problematic.The George W. Bush administration had favored a substantially broadened concept of anticipatory self-defense. This sweeping American doctrine asserted that traditional notions of deterrence could not be expected to work against a new kind of enemy. “We must,” stated The National Security Strategy for the United States of America (September 20, 2002), “adapt the concept of imminent threat to the capabilities and objectives of today’s adversaries.” The 1837 incident from which the modern legal concept of anticipatory self-defense is drawn (The Caroline), required a threat that is “imminent in point of time.”In actual practice, this “adaptation” meant nothing less than striking first against presumptively dangerous adversaries whenever deemed necessary. In any plausible comparison to the dangers Israel currently faces from Tehran, however, the alleged risks to the United States from Saddam Hussein’s Baghdad in the wake of 9/11 must appear vague and uncertain. When it is finally understood in terms of Israel’s present concerns about an overtly genocidal Iran, any Israeli strategy of anticipatory self-defense should be less subject to proper jurisprudential doubt than was America’s Operation Iraqi Freedom.In the post-Holocaust and post-Nuremberg international system, the right of individual states to defend themselves against genocide is overriding. This right does not stem directly from the language of the Genocide Convention, which does not explicitly link genocide to aggressive war, but it can still be extrapolated from (1) the precise legal language of anticipatory self-defense, including the 1837 case of The Caroline; and (2) all subsequent authoritative reaffirmations of law identifiable at Article 38 of the Statute of the International Court of Justice. The right of anticipatory self-defense to prevent genocide can also be deduced from certain basic principles of self-protection codified at the 1969 Vienna Convention on the Law of Treaties, and, more generally, from the confluence of persistently anarchic international relations with now-obligatory legal norms of basic human rights.Should Israeli decision-makers determine that they do have a right to act first against Iran to prevent genocidal aggression, any resultant Israeli resorts to preemptive force would still have to be consistent with the laws of war of international law, or the law of armed conflict.Strategic circumstances and the consequences of strategic surprise have changed a great deal since The Caroline, thereby greatly and sensibly expanding legal grounds for anticipatory self-defense. Today, in an age of chemical/biological/nuclear weaponry, the time available to any vulnerable state under attack could be only a matter of minutes. From the special standpoint of Israel, soon about to face a potentially annihilatory Iran armed with nuclear weapons, a suitably hard-target resort to anticipatory self-defense could be both lawful and law-enforcing. Again, whether any such eleventh-hour preemption would also make operational sense is another question altogether.Before the start of the Atomic Age, any justification of anticipatory self-defense would have to have been limited to expected threats of aggression from other states, not genocide. Today, however, the conceivable fusion of nuclear weapons capacity with aggression could transform war into genocide. Although there are no true precedents of resorting to preemption as a law-enforcing means of preventing genocide or “conspiracy to commit genocide” by one state against another, the pertinent right to such pre-attack self-defense is rooted, inter alia, in The Caroline.If it was already legal, long before nuclear weapons, to strike preemptively in order to prevent entirely conventional aggressions, how much more permissible must it be to strike preemptively to defend against a potentially genocidal nuclear war? Law can never compel a state to wait until it has absorbed a devastating or even genocidal first strike before acting to protect itself. Both the Security Council and the General Assembly correctly refused to condemn Israel for its 1967 preemptive attacks. Incorrectly, however, whether or not it had then accepted the existence of a formal state of war between Israel and Iraq – a condition of belligerency that was openly insisted upon by Baghdad – the UN did condemn Israel for Operation Opera in 1981. This jurisprudentially wrongful condemnation was the result of pure realpolitik or power politics.Present-day Israel is engaged in a condition of protracted belligerency with Iran. Again and again, Tehran has implied that there exists a “state of war” with Israel.Once faced with an already-nuclear Iran, Jerusalem’s only remaining strategic options would center upon some still-practicable combination of active ballistic missile defense, and nuclear deterrence. The resulting condition of mutual nuclear vulnerability could resemble earlier Cold War images of “two scorpions in a bottle,” the metaphoric description first created by physicist J. Robert Oppenheimer. It would likely become a “fusion” of mutual uncertainty and radical instability, a potentially explosive posture more unpredictable than earlier US-Soviet conditions of Mutual Assured Destruction (MAD). Above all, perhaps, this is because of the heightened probability of Iranian leadership irrationality.Iran’s ongoing stance toward Israel remains unequivocally genocidal. Because international law is not a suicide pact, Jerusalem, now facing a fusion of enemy nuclear capacity with enemy criminal intent, reserves every reciprocal right of national self-protection. This includes the right to anticipatory self-defense.In the end, Israeli calculations of genocide prevention from Iran would have to display pragmatic as well as legal components. Any Israeli decision to preempt against Iran would have to be based not only upon a due conformance with the rules of applicable law, but also on overriding strategic and operational expectations.Even if Israel were to fully accept the lawfulness of anticipatory self-defense against Iran, it would act accordingly only if such a complex defense were also expected to work.To be sure, Israel would be acting lawfully by introducing a General Assembly resolution calling for Iran’s expulsion from the United Nations. Although any such resolution would carry a high probability of failure, the corresponding diplomatic efforts could nonetheless underscore that several governments in Tehran have remained unabashedly genocidal in their policies toward Israel. Also worth noting is that any such policies, to be illegal under international law, do not need to actually be implemented. Rather, it is enough that they incite to genocide, or display genocidal intent.The author was educated at Princeton and is a professor of international law at Purdue University. He publishes widely on international law and Israeli strategic defense issues.