The governments of the United States and Canada say they have evidence that an Iranian missile — rather than mechanical failure or a terrorist’s bomb — brought down a Ukraine International Airlines jet shortly after takeoff in Iran on Wednesday, killing all 176 persons aboard. Video shows the moment the missile collided with the plane above Tehran. And Iran on Friday night also finally admitted blame, although it chalked the tragedy up to "human error."

This is not the first time such a thing has happened. In 2014, a Russian missile killed 298 people aboard a Malaysian airliner over eastern Ukraine. In 1988, a U.S. warship shot down an Iranian aircraft over the Gulf, killing 290 aboard, after thinking it was an attacking Iranian fighter plane.

Still, there is insufficient information to conclude that this was an unspeakable war crime — the deliberate targeting of civilians — or instead, a merely tragic accident as Iran now claims. Perhaps it was something in between. What is certain, however, is that violations of international law led to this tragedy.

Several realms of international law are at play here. Jus ad bellum is the Latin term commonly used to describe the rule that states may not use, or threaten to use, force in their international relations. This is a natural and necessary rule to protect each state’s sovereign rights. It is accepted and asserted by all nations and was codified in international law with the drafting of the United Nations Charter in 1945.

There is insufficient information to conclude that this was an unspeakable war crime — the deliberate targeting of civilians — or instead, a merely tragic accident as Iran now claims. Perhaps it was something in between. Gabor Rona Law professor

There are three exceptions to this prohibition. The first exception is when the force is conducted by the state within the state’s boundaries, with no violation of sovereignty. (The attacks must still comply with other international human rights law or law of armed conflict, discussed below.) Since the missile was launched by Iran in its own airspace, the consent issue is irrelevant. Consent, however, is indeed an issue concerning last week’s U.S. drone strike that killed several people, including Iranian commander Qassem Soleimani at the Baghdad airport. Iraqi assertions that the U.S. conducted the drone strike without Iraqi consent lie at the heart of the Iraqi Parliament’s vote to expel American forces from the country.

A second exception to the prohibition of the use of force, inapplicable here, is when force is authorized by the U.N. Security Council, for example in peacekeeping operations.

But the third exception is important. The prohibition does not apply to acts of self-defense in response to an attack. This exception, also written into the U.N. Charter, provides most of the fodder for debate about the recent hostilities between the U.S. and Iran.

By now, most people are familiar with the notion that the right of self-defense applies only in response to, or in the event of “imminent threat.” What has received less airplay, however, are the elements of an imminent threat and other conditions that apply to uses of force. To be imminent, a threat must be so direct as to provide no option but the use of force. An unspecified threat that may materialize in the future doesn’t count. Even then, the force must be reasonably calculated to ameliorate the threat.

Killing a government official is not permitted if it is not likely to stop or prevent an attack. Thus, the U.S. targeting of Soleimani is likely unlawful, not only because no imminent threat has been identified, but also because even if one was identified, it is difficult to see how Soleimani’s death would end the threat. He has been replaced and his forces can still act.

Bringing down a civilian airplane is even more obviously prohibited for similar reasons. Where force is permitted in response to an attack or an imminent threat, it must also be proportional to the attack or threat posed. You can’t destroy the interior ministry and kill all its employees because the minister of interior leads a force that throws rocks at your embassy wall.

Finally, acts of revenge are not self-defense. The claim that the “blood of U.S. forces” is on Soleimani’s hands is not a justifiable reason for an assassination — and neither are Iranian promises to avenge his death. Were it otherwise, endless cycles of revenge would make the world a considerably more dangerous place than it already is.

Acts of revenge are not self-defense. The claim that the “blood of U.S. forces” is on Soleimani’s hands is not a justifiable reason for an assassination — and neither are Iranian promises to avenge his death. Gabor Rona Law professor

The second realm of international law at play is the law of war. In war, people may be targeted not only based on their conduct, but also based merely on their status as enemy combatants. For this reason, it is crucial to avoid asserting the laws of war where war does not exist. The “war on terrorism” is not a real war. Wars are fought by proper nouns, like Germany, Japan, al Qaeda and ISIS: organized entities with command structures, capable of complying with laws of war (whether they do so or not) and capable of being defeated and promising not to do it again. “Terrorism” is a common noun that fits none of these criteria. That’s not to say acts of terror cannot create, or be present in, a state of war. And though Iran may rightfully be classified as a “state sponsor of terrorism,” that fact alone does not create a state of war.

And even in a real war, there are limitations on the use of force. Only combatants may be targeted, precautions must be taken to avoid civilian casualties, and even attacks against combatants are prohibited if they create an unreasonable risk of disproportionate civilian harm (“collateral damage”). Thus, even if Iran deemed itself at war — a questionable assertion — deliberate targeting of a civilian airliner would be a heinous war crime.

But what if it was an accident, or a mistake, as President Donald Trump seems to be suggesting in the case of the Ukrainian plane? If the attack was undertaken without sufficient precautions to make sure the target was a military object, that too would be a war crime in armed conflict.

In the absence of armed conflict and the application of the laws of war, a third realm of international law prevails: human rights law. There are a couple of important differences between law of war rules and human rights law rules. One, already mentioned, is that in war, killing can be justified based on a person’s status as the enemy, while in the absence of war, human rights law permits killing only on the need to act in self-defense. In war, I can shoot my enemy while he is eating in his barracks. Outside of war, I can only shoot a person who poses a threat to life that cannot be stopped otherwise.

Another important difference between peace and war is that in war, lawful combatants may not be punished for their lawful actions, which include targeting of enemy forces. This is another reason why the “blood on his hands” argument about Soleimani falls flat. The Gulf War is long over and those who fought in it, whether Iraqi, Iranian or American, broadly violated no international law in targeting their enemy. While more recent acts of terrorism allegedly organized by Soleimani violate international law, measures short of assassination, such as sanctions and criminal prosecution, by an international tribunal if necessary, must take precedence. Even if such measures are unavailing, the notion that any state can kill terrorists because their home country is “unwilling or unable” to bring them to justice is questionable.

None of what I’ve written here concerns the wisdom of any of these resorts to force, only their legality. But as a practical matter, whether or not they violate international law, civilian casualties are common, always tragic, and often preventable. Iran might have deliberately targeted a civilian object or simply committed a tragic error. We may never know. Even if it’s the latter, Iran likely violated international law, as has the United States. There are various ways to determine responsibility. One way, perhaps posing a moral rather than legal question, is to ask whether the 176 souls who perished on that plane would still be alive had the United States not sought to kill Qassem Soleimani.

Gabor Rona is a law professor at Cardozo Law School, the former international legal director of Human Rights First, and a former legal advisor in the legal division of the International Committee of the Red Cross.

This piece was first published by NBC Think.

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