We should have seen it coming. On January 3rd, United States armed forces killed Qassem Soleimani, the commander of the Quds Force of the Islamic Revolutionary Guard Corps, which is part of Iran’s armed forces. That same day, Iran’s ambassador to the United Nations submitted a letter to the U.N. Secretary General and the President of the U.N. Security Council, describing the U.S. strike as “State terrorism,” “a criminal act,” and “a gross violation of the fundamental principles of international law,” among other things.

The letter did not, however, describe the U.S. strike as an armed attack against Iran, triggering Iran’s right of self-defense under the U.N. Charter. True, the letter said that Iran “reserves all of its rights under international law to take necessary measures in this regard, in particular in exercising its inherent right to self-defense.” But this reservation of rights was followed by an appeal to the U.N. Security Council to “uphold its responsibilities,” foremost among them maintaining international peace and security. The letter did not indicate that Iran planned to use armed force unilaterally.

By the morning of January 7th, Iran’s legal position had changed. In one interview, Iran’s Foreign Minister, Mohammad Javad Zarif, again described the U.S. strike as “state terrorism.” But Zarif then added:

This is an act of aggression against Iran and amounts to an armed attack against Iran, and we will respond. But we will respond proportionally not disproportionally. . . . We will respond lawfully, we are not lawless people like President Trump.

In a separate interview, Zarif said:

This was an act of aggression, an armed attack, albeit a cowardly armed attack, against an Iranian official in foreign territory. It amounts to war, and we will respond according to our own timing and choice. . . .

[I]n exercising our right to self-defense, we are only bound by international law, unlike the United States, which is not bound by international law.

Describing the U.S. strike as “an act of aggression” implicates the collective responsibility of the Security Council to “decide what measures shall be taken … to maintain or restore international peace and security.” In contrast, describing the U.S. strike as “an armed attack against Iran” implicates Iran’s unilateral right to use armed force in self-defense against the United States. This was an ominous sign.

Less than 24 hours later, Iranian armed forces launched more than a dozen missiles at military bases in Iraq hosting U.S. forces. In a sign of the times, Zarif confirmed Iran’s legal position by tweet:

Iran took & concluded proportionate measures in self-defense under Article 51 of UN Charter targeting base from which cowardly armed attack against our citizens & senior officials were launched.

We do not seek escalation or war, but will defend ourselves against any aggression.

In fact, these measures were not proportionate, because they were not in self-defense. The right of self-defense permits the use of force only to halt or repel an armed attack that has already begun, remains ongoing, or (perhaps) is imminent. These measures were armed reprisals, retaliation for an armed attack that was clearly over. As such, these Iranian strikes were unlawful, for the same reason (or one of the same reasons) that the U.S. strike was itself unlawful.

This is not simply one view of international law. It is Iran’s. Or, at least, it was Iran’s. During an earlier dispute with the United States, before the International Court of Justice, known as the Oil Platforms case, Iran claimed that

The principle of necessity means that only that use of force which is necessary in order to repel an attack constitutes lawful self-defense. If an armed attack is terminated, there is no further need to repel it. Thus, self-defense is limited to an “on-the-spot reaction”, i.e., the necessary, immediate response to an armed attack. This is the principle of immediacy: it means that the employment of counter-force must be temporally interlocked with the armed attack triggering it. In the case of the invasion of another State’s territory, in principle an attack still exists as long as the occupation continues. But in cases of single armed attacks (as distinguished from a general situation of armed conflict), the attack is terminated when the incident is over. In such a case the subsequent use of counter-force constitutes a reprisal and not an exercise of self-defense.

Iran acknowledged that “[t]he distinction between lawful self-defense and unlawful reprisals is not . . . free from difficulty,” but noted that the delayed timing of a reprisal and its premeditation typically reveals its punitive (rather than protective) character. Last night’s operation was apparently premeditated for several days following the U.S. strike, just as the U.S. strike was apparently premeditated for quite some time. Both were unlawful reprisals.

In its submissions to the Court, Iran allowed that

There may be circumstances in which the victim State has experienced a series of attacks, and apprehends further attacks, so that the measures taken, although taken after the last actual attack are designed to protect the State against future attacks. An illustration would be in the destruction of bases from which attacks had occurred in the past, and from which future attacks were anticipated. But, in general, this view of self-defense had been rejected in Security Council practice and rightly so, because the apprehended future attacks, if not imminent, are hypothetical; and in any event the measures tend to be designed to “teach a lesson”, to inflict retribution and to deter only by demonstrating that aggression does not pay.

Zarif suggested that Iran targeted the base from which the U.S. strike was launched, but did not suggest that the same base would have been used to launch future attacks against Iran. In any event, the Iranian strikes seem precisely “to be designed to ‘teach a lesson’, to inflict retribution and to deter only by demonstrating that aggression does not pay.” By Iran’s own standard, last night’s operation was unlawful.

As of this writing, it seems that the cycle of retaliation has paused, at least for a time. Iran indicated that it will not launch further strikes unless the United States launches further strikes of its own. This proposal does not inspire much confidence.

Iran had it right the first time. It is the responsibility of the Security Council to maintain international peace and security. Since the United States and Russia sit on the Security Council, wielding vetoes over any proposed measure, the prospects for peace may appear dim. But given the abject failure of unilateral force to achieve anything of lasting value, collective security measures seem worth trying. Exchanging armed reprisals can only lead to ruin.