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.