President Joe Biden prepares to sign a series of executive orders at the Resolute Desk in the Oval Office just hours after his inauguration on January 20, 2021 in Washington, DC. Biden became the 46th president of the United States earlier today during the ceremony at the U.S. Capitol. (Photo by Chip Somodevilla/Getty Images)

The United States carried out airstrikes in Syria early Friday morning, killing several people and destroying several buildings. The Pentagon says that the airstrikes were a response to a rocket attack that occurred on Feb. 15, some 10 days earlier, at Erbil airport in northern Iraq, some 400 km away. That rocket attack killed a Filipino contractor, wounded four American contractors, and wounded a U.S. soldier.

It’s not clear whether the U.S. airstrikes targeted the group responsible for the rocket attack, or other groups affiliated with it. The Pentagon says “the strikes destroyed multiple facilities located at a border control point used by a number of Iranian backed militant groups including Kait’ib Hezbollah and Kait’ib Sayyid al Shuhada.” It did not mention Awliya al Dam, the group that claimed responsibility for the rocket attack in Erbil.

The U.S. airstrikes almost certainly violated international law, for two basic reasons. The airstrikes did not repel an ongoing armed attack, halt an imminent one, or immediately respond to an armed attack that was in fact over but may have appeared ongoing at the time (see here and here). And the airstrikes were carried out on the territory of another State, without its consent, against a non-State actor (or two, or more) (see here). These two reasons, combined, are decisive. It cannot be lawful to use armed force on the territory of another State when it is clear that no armed attack by a non-State actor is ongoing or even imminent.

The Pentagon says that the attacks were launched “in response to recent attacks against American and coalition personnel in Iraq, and to ongoing threats to those personnel.” The Feb. 15 attack was clearly over and not ongoing. And ongoing threats are not imminent attacks. The United States is free to take lawful action in Iraq to improve the long-term security of its forces and contractors in Iraq. It may not legally take military action in Syria to improve the long-term security of its forces and contractors in Iraq.

The U.S. government has not yet explained its decision to strike inside Syria rather than take lawful action inside Iraq. According to one former official, “[t]he decision to strike in Syria instead of Iraq was likely to avoid causing issues for the Iraqi government.” Needless to say, it is unlawful to bomb one country to avoid “causing issues” with another.

A few days ago, the State Department said about the rocket attack in Erbil that “we will respond in a way that’s calculated within our own timetable and using a mix of tools at a time and place of our choosing.” That is not how international law works. The use of armed force is lawful only if, when, and where it is necessary. The U.S. government appears to concede that it was not necessary to strike inside Syria. It was merely convenient.

The Pentagon says the airstrikes were a “proportionate military response” to the rocket attack in Erbil. International law permits a proportionate military response to an ongoing armed attack, or perhaps an imminent one. A proportionate military response to a previous armed attack, that is clearly over, is not proportionate self-defense. That is an armed reprisal. And even proportionate armed reprisals are illegal (see here and here).

The U.S. airstrikes were not defensive. They were expressive. The Pentagon says that the operation “sends an unambiguous message: President Biden will act to protect American and coalition personnel.” The operation sends another message: President Biden will violate international law, much like his predecessors.