Rebecca Ingber explains why it matters very much whether there was an “imminent” Iranian attack earlier this month:

The framers gave Congress, not the president, the power to declare war with the understanding that it would slow the rush into conflict. A narrow exception for circumstances in which there is truly no time to go to Congress for a vote makes sense. But the president cannot circumvent Congress simply because he views it as good policy to take action.

After hiding behind the claim that they acted to head off an “imminent” attack, the Trump administration now shrugs and claims that it makes no difference if there was such an attack in the works or not. Of course, as far as the legality of the attack is concerned, it makes all the difference in the world. If there really were an “imminent” attack on U.S. forces, the president would be permitted to order military action to avert it. When there is no evidence at all that such an attack was in the offing, the president is obliged to seek Congressional approval first. The president is not free to shout “self-defense” and then initiate hostilities against another state.

Assassination as a tactic is itself prohibited. Charli Carpenter explains:

First, as a high-ranking official of an actual government, he cannot as easily be cast as a terrorist renegade as nonstate actors like bin Laden. Second, in wartime, a military official such as Suleimani could arguably be lawfully killed but only if an international armed conflict already existed between Iran and the United States. And even then, it would not be legal to single him out as an individual, least of all in a third country not party to the war.

Ingber also makes an important point that the location of the attack compounds the illegality of the strike, because the action was taken on Iraqi soil without their government’s permission:

Iraq, of course, did not itself attack us. A crucial step in determining whether it is necessary to use force on the territory of a state that did not itself attack us — the long-standing U.S. approach, dating to the Caroline incident, increasingly adopted by other states — has been to, first, establish the imminence of a forthcoming attack and then to analyze whether that state is itself unwilling or unable to prevent or stop the attack. The administration has not put forward evidence on either question — and has not even addressed the issue of Iraqi sovereignty.

In short, the president did something illegal by ordering the strike without Congressional approval. It was also illegal because he ordered a prohibited assassination. Finally, it was illegal because he ordered an attack in the territory of another country without that government’s approval. Trump had no authority to do what he did, and he made a mockery of the Constitution and international law by doing it. To top it off, he and his top officials have spent the last two weeks lying about why they did it. We know that the president doesn’t care if what he does is legal, and he doesn’t respect the limits on presidential power contained in the Constitution. The attack that the president ordered two weeks ago was also an attack on the rule of law. The question before us is whether enough of us still care about flagrant presidential lawbreaking to oppose him when he orders illegal attacks.