The president announced on June 21 that he had called off a potential U.S. military strike on Iran in response to Iran’s shootdown of a U.S. Navy remotely piloted vehicle (RPV). The strike, according to the president, could have incurred casualties of as high as 150 people—information that has sparked discussion over the proportionality of such a response under international law. Before jumping to this debate, however, there is another issue that needs to be considered first: the question of necessity.

In one of the great scenes from the movie “Anatomy of a Murder,” defense counsel Paul Biegler is asked to defend Lieutenant Manion, charged with murder for shooting his victim, Barney Quill, at point-blank range after Manion’s wife told him Quill raped her. Biegler meets with Manion and tells him the facts don’t support a justification defense. Manion erupts, “Why? Why wasn’t I justified killing the man who raped my wife?” Biegler responds, “Time element. If you had caught him in the act you would have been justified. But you didn’t; you shot him later. That’s murder, premeditated and with vengeance” (emphasis added).

Whether considering self-defense (or defense of others) in the domestic or international context, Biegler’s explanation highlights one of the most important limitations on such a claim of justification: It may never be invoked to justify an act of revenge in response to an unlawful threat that is no longer ongoing or imminent. This is a key component of self-defense necessity in any context, and it reflects that the legal justification to engage in conduct that would otherwise be unlawful begins—and ends—with genuinely necessary self-protection. More specifically, Biegler educated his client that the law justifies action taken for self-help in response to an unlawful threat or act of violence only to prevent or terminate that threat, not to punish the assailant or take revenge.

One would expect that a similar discussion occurred within the U.S. government regarding the planned strike against Iran. While the context was unquestionably different, the key principle Biegler explained to Manion—that self-defense never justifies an act of retaliation once the unlawful threat no longer exists—is just as relevant as it was in “Anatomy of a Murder.” The U.S. framed the strike in the language of self-defense. But given that the strike was responding to Iran’s shootdown of the Navy RPV, which was already over by the time the strike would have taken place, is this self-defense argument legitimate?

In the context of international law, there are certainly situations when military action based on an asserted justification of self-defense will lawfully occur after an unlawful attack. The critical inquiry in such situations is whether a use of force conducted after an unlawful armed attack is legitimately necessary to protect against continuing unlawful violence or was instead an act of retaliation or vengeance. This is an especially complicated aspect of assessing compliance in the domain of international security and law. In the international domain, unlike the domestic individual self-defense situation presented in the movie scene, it is not necessarily unreasonable for a victim state to assess an attack as an initial foray into a broader aggressive operation or campaign. In such situations, a proportional act in response to an initial attack may eliminate or deter the reasonably anticipated ongoing threat and, thereby, fall within the scope of self-defense necessity.

President Trump’s aborted plan to respond to the Iranian attack on the U.S. RPV is certainly not the first time that the U.S, or another state, has invoked the inherent right of self-defense to justify what appear to be retaliatory strikes. Consider the U.S. attacks against Libya in response to the 1986 Berlin discotheque bombing or against Iraq in response to the failed 1993 attempt to assassinate former President George H.W. Bush. The 1989 U.S. invasion of Panama provides an even more compelling example. According to a U.S. General Accounting Office report, the U.S. State Department pointed to the defense of U.S. nationals and military personnel in Panama as among the justifications for the invasion that toppled General Manuel Noriega’s regime. In the abstract, that claim is not controversial. However, this assertion of self-defense was triggered strictly by two isolated incidents of Panama Defense Forces (PDF) violence against off-duty U.S. military personnel, including one U.S. service member who was shot and killed while evading a roadblock. It’s difficult to see a pattern of future violence based on those two instances that made it necessary to invade in order to prevent such future acts.

Setting aside the question of proportionality, it is easy to appreciate how readily this theory of international legal justification to an anticipated ongoing threat of unlawful violence can be exploited as a subterfuge to engage in retaliatory strikes. In Panama, was it reasonable for the United States to treat these two incidents as a justification for self-defense military action? Or were these incidents exploited to justify an otherwise unlawful invasion of another sovereign country? And while the scale of the aborted attack on Iran was almost certainly nothing like the invasion of Panama (at least let’s hope not), scale is really a secondary issue in relation to legality. The first and essential issue is the same one Paul Biegler had to explain to Lieutenant Manion: Was the use of force a necessary measure to repel an act of unlawful violence, or had the use of force become unnecessary because that act of unlawful violence had terminated and was not the opening salvo of a broader campaign?

There is no easy answer to this question. Even in Panama, it was not implausible for President Bush to view the two acts of PDF violence as an indication that the Noriega regime had decided to cross a line of continuing violence after nearly two years of tense standoff. (It is ironic that the motto of the U.S. Army in Panama was “no ground to give,” while the motto of the PDF was “ni un paso atrás,” meaning “not one step back.”) If that were true, the risk of hesitation to the tens of thousands of U.S. citizens living in and around Panama City was substantial. Had a broader campaign of violence been unleashed, it would have been nearly impossible for the U.S. military to protect all U.S. citizens in that country.

Such critiques will always be frustrated by the inevitable secrecy that cloaks government decision-making: Without access to the information relied on to justify such action, it is hard to know why a government may have assessed what appears to have been a one-off incident to constitute a much more ominous indication of further imminent violence. Perhaps the very nature of attacks that bear the hallmarks of vengeance necessitates greater transparency regarding the intelligence and other indicators that ostensibly provide that justification. While such fuller disclosure by a state is not likely to be viewed as legally obligatory, from both a deterrence and a legitimacy perspective it seems that the state that engages in attacks that seem so outwardly retaliatory in nature owes the public more than just an invocation of self-defense.

The conversation about the proportionality of the proposed strike is certainly important. But these situations demand more scrutiny on the predicate question of self-defense necessity in order to clarify the line between legitimate justification and unjustified revenge.