“The fierce revenge by the Revolutionary Guards has begun,” Iran’s Islamic Revolutionary Guard Corps announced after Iran launched over a dozen ballistic missiles against two U.S. military bases in Iraq late Tuesday evening. This attack takes place against the backdrop of Iranian leadership’s promises of forceful retaliation for the U.S. drone strike in Iraq last week that killed General Qassem Soleimani, Iran’s leading military figure. Indeed, Iran’s senior most officials have been promising revenge for days, with no claim that they anticipated future U.S. strikes. More specifically, within hours of the U.S. strike last week, Iran’s Supreme Leader Ali Khamenei vowed “forceful revenge” for General Soleimani’s death; Iran’s United Nations ambassador characterized the U.S. attack as “an act of war” and repeated Khamenei’s threat, crowing that Soleimani’s death would be met with “revenge, a harsh revenge.” Gen. Hamid Sarkheili of the Iranian Revolutionary Guard told a crowd of Soleimani mourners on Monday that, “[w]e are ready to take a fierce revenge against America…American troops in the Persian Gulf and in Iraq and Syria are within our reach.” And as if to punctuate their motivation, the Iranian missile attack was accompanied by this familiar refrain of “fierce revenge.”

Iran may think it is justified in what it calls revenge, but its actions and rhetoric are fundamentally inconsistent with international law, ironically the very law Iran invoked to condemn the U.S. attack. Revenge (often called retaliation) is not a lawful basis for a State’s use of armed force. Instead, international law permits a State to use force against another State (or in the view of many, including the United States, non-state organized armed groups) only when necessary to defend against an imminent, actual, or ongoing unlawful armed attack, or pursuant to a United Nations Security Council resolution. Neither of these bases justify revenge, retaliation, or reprisal; and neither seemed to justify Iran’s threats or attack. Indeed, despite the rhetoric Iran appears to actually understand this, which likely explains why following the missile attack Iran’s Foreign Minister posted on Twitter that the country “took & concluded proportionate measures in self-defense” (contradicting nearly a week of threats of revenge).

Invoking the rhetoric of self-defense does not ipso facto justify a State’s use of military force absent a reasonable basis to conclude the State faced one of the triggering justifications for such necessary self-help action. This applies equally to the United States and Iran, both of which have now launched attacks that could easily be viewed as acts of retribution. Accordingly, if Iran did what it actually promised – launch a military attack to retaliate for the U.S. Soleimani strike and not based on an imminent threat of armed attack by the U.S. — Iran has, paradoxically, engaged in the same illegality it has been condemning.

Self-defense on the international level, like self-defense in any other context, is a legal justification that requires the use of force to be absolutely necessary to protect against an imminent threat of unlawful violence. If that act of violence is completed, this self-help justification expires, unless the victim reasonably perceives an ongoing threat. This “timeliness” aspect of self-defense necessity functions to prohibit a victim of unlawful violence from transforming a genuine self-protection justification into a justification to take revenge.

That is, a U.S attack purely in retribution for earlier Iranian attacks is squarely prohibited by international law, specifically the United Nations Charter. Like an act of self-defense in the individual context, in which responses to and retribution for past violent acts is ceded to the criminal justice system, international law cedes legal authority for enforcement of international law – including violent punishment of aggressors – to the U.N. Security Council. Though greatly embryonic compared to domestic law enforcement systems, and often hobbled by the impact of the veto power vested in the five permanent members of the Security Council (including the United States), this international legal structure with all its limits and flaws remains the primary (if not exclusive) means by which a State responsible for a completed act of unlawful aggression is subjected to sanction.

The core of this international superstructure adopted to limit situations when States may legitimately use military force to protect their interests is the presumptive prohibition against the use or threatened use of force by States laid out in Article 2(4) of the U.N. Charter:

“All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations.”

Just as the use of individual force has long been domestically illegal – criminal – as a means of resolving disputes or for any reason, the U.N. Charter applies this proscription to States, prohibiting military force (or its threat) as an instrument of international relations.

But like any society, this prohibition cannot guarantee compliance. Accordingly, if and when a State is a victim of an actual or imminent unlawful armed attack it may, pursuant to Article 51 of the U.N. Charter, take necessary self-help military action to protect itself, and must notify the Security Council of such action to give it an opportunity to take further action. And again, mirroring domestic law, this self-help exception to the prohibition against the use force comes with three customary requirements: imminence, necessity, and proportionality. Accordingly, once the threat is terminated it must turn to the Security Council to authorize any subsequent use of force needed to restore or maintain international peace and security. It cannot simply decide to exceed the necessity of self-help and engage in its own revengeful or punitive military attacks against the aggressor state.

A victim of unlawful conduct, be it an individual or a State, is simply not legally justified in using force to “punish” or “sanction” the unlawful aggressor, whether it characterizes its action as revenge, reprisal, or anything else. While such actions of reprisal or revenge may be appealing to many and even generate public empathy by the aggrieved party, a society built on the rule of law limits the authority to engage in self-help violence to only those situations where it is necessary to protect the victim and restore the status quo ante.

Applying this legal equation to current events is complicated by the limited access to the intelligence and other information that ostensibly informed the U.S. decision to launch the attack on Soleimani. Other commentators quickly concluded the U.S. acted illegally, just as many will make the same assertion about Tuesday night’s attack by Iran. In contrast, we simply refuse to assume we know enough at this point for such certitude. But we do know that U.S. officials (unfortunately with an exception of our President) have been using all the right terminology to make the case that the Soleimani strike was a lawful response to an imminent unlawful threat, and now Iran seems to be backtracking in an effort to do the same. We also know that both States can now point to prior recent attacks to bolster their assertions of self-defense. However, while these prior incidents are certainly relevant to assessing future intentions and capabilities, they did not, standing alone, provide a legal justification for Iran or the U.S. to launch attacks.[1] However, this record was certainly relevant to the assessment of intelligence indicating an adversaries capabilities and whether more was about to come. It’s important to monitor when, for example, the deaths of 608 Americans in Iraq – which occurred between 2003 and 2011 – are being attributed to Soleimani for the valid purposes of this kind of intelligence assessment or for an invalid understanding of the legal basis for the use of force. Along these lines, rhetoric contributes little to a meaningful assessment of self-defense legitimacy and may often contribute to claims of invalid revenge. For example, assertions by current (and former) U.S. officials that Soleimani has the blood of hundreds of American troops on his hands may be understandable as they seek to highlight the extent of his involvement with actors hostile to U.S. forces and interests, but contributes to the perception that the attack was more about revenge than self-defense. Scrutiny of the factual basis for the claim of self-defense should therefore be a key focus of Congressional efforts to ensure this high-stakes decision on the administration’s part complied with the law we as a nation and our military champion.

This is why we believe it is so essential that the U.S. administration articulate to the American people and the broader international community a compelling case that it made a reasonable and credible assessment that its Soleimani attack was necessary to prevent another unlawful attack on U.S. military personnel or U.S. facilities – and that such projected attack was imminent, leaving no reasonable time for non-forceful measures to obviate such threat.

It is also why it is per se illegal for Iran to threaten the use of military force to take revenge, even if they pretend to demonstrate respect for international law by emphasizing they will only attack U.S. military targets in a “proportional” way. And it is why Iran and the United States should be forthcoming with the information that led to their respective asserted determinations that their attacks were necessary to prevent subsequent imminent uses of force by their antagonist.

In contrast to Iran’s near-constant refrain of revenge as its basis for a threatened strike – despite claiming that Tuesday’s attack was lawful self-defense – on the other side of the world the Trump Administration has been consistently claiming its strike last week was indeed internationally lawful as an exercise of the inherent right of self-defense. The U.S. immediately invoked the inherent right of self-defense as the principal U.S. legal authority to justify its drone strike in Iraq targeting General Soleimani. The United States attacked the general because, per the State Department, he was planning “imminent attacks against American personnel and facilities in Lebanon, Iraq, Syria and beyond.” Secretary of State Pompeo explained that Americans “are safer in the region” after the U.S. drone strike, because Soleimani’s anticipated actions involved an “imminent attack” that “would have put hundreds of lives at risk.” The Chairman of the Joint Chiefs of Staff, General Milley, affirmed that the U.S. had “the intelligence I saw– that was compelling, it was imminent, and it was very, very clear in scale, scope..” And President Trump claimed the morning after the strike that, “[w]e took action last night to stop a war, we did not take action to start a war,” and he too called the Soleimani’s threat of an attack “imminent.”

The stakes involved in the U.S. military strike and the escalation we now know it generated implicate a wide array of diplomatic, political, and strategic considerations. It is therefore logical and appropriate to scrutinize the U.S. claim of self-defense justification, something that began almost as soon as the attack was executed. And the Iranian missile strike of Tuesday evening should be subjected to the same scrutiny.

Geoffrey S. Corn, a retired U.S. Army JAG officer, is the Vinson & Elkins Professor of Law at South Texas College of Law Houston and a Distinguished Fellow for the Jewish Institute of National Security for America’s Gemunder Center for Defense and Strategy.

Rachel E. VanLandingham, a retired U.S. Air Force JAG officer, is a professor of law at Southwestern Law School, Los Angeles and a center expert at the Jewish Institute of National Security for America’s Gemunder Center for Defense and Strategy.

Both authors served more than 20 years and both began their military careers as regular line officers before attending law school and serving as military lawyers.

Image: President Donald Trump speaks from the White House on January 08, 2020 addressing the Iranian missile attacks that took place on the previous night in Iraq (Win McNamee/Getty Images)

[1] The one qualifier to this is the possibility that these attacks are occurring in the context of an ongoing armed conflict between the United States and Iran. While such an armed conflict must be assessed factually, it does seem relevant that neither the United States nor Iran seem to be invoking this theory of legality. Instead, each side seems to be treating its attacks as a “one off” measure, whether based on an assertion of self-defense or revenge. Certainly these attacks themselves qualify as armed conflicts subject to law of armed conflict regulatory rules. What is far more complicated is whether each attack qualifies as a distinct armed conflict terminating when the attack terminates, or whether there is now an ongoing armed conflict? Because both the U.S. and Iran have invoked the self-defense as the justification for each action, our analysis is limited to the invalidity of revenge or reprisal as a justification for such actions.