Vox features an interview today with constitutional lawyer Stephen Vladeck:
As a matter of US law, was the latest American military strike on Syria legal?
Almost certainly not. To be legal, the strike would have to authorized either by some act of Congress or by the president’s own powers under Article II of the Constitution. And neither of those conditions appear to have been met here.
I’ve seen lots of versions of this opinion, but it’s wrong. In dorm-room-bull-session terms there might be something to it, but in practical terms there isn’t. If an action is approved, either implicitly or explicitly, by the executive, the legislative, and the judicial branches, then it’s constitutional. That’s how our legal system works. Full stop.
In this case, the executive obviously approved the action. Congress has had many opportunities to rein in these kinds of strikes, and they haven’t. Ditto for the Supreme Court, which has always given the president wide latitude in matters of military force.
Until this changes, lobbing missiles at anyone we want is constitutional.