Marty Lederman spells out  why the attack on Syria is illegal under international and domestic law:
In this case, however, Congress has not authorized the attacks on Syria, or otherwise made the solemn decision that the U.S. should breach the Charter. Therefore, not only has President Trump put the U.S. in breach of its treaty obligations — in violation of his Article II obligation to take care that the treaty is faithfully executed — but he has also likely violated the constitutional allocation of war powers, too.
This isn’t the first time that an American president has ordered military action that violates the U.N. Charter, but that doesn’t make it any less illegal now. Clinton did this in 1999 when he ordered the bombing of Yugoslavia, but because it was deemed a “good” intervention its blatant illegality was ignored. Unfortunately, the mostly positive reaction to Trump’s decision to attack the Syrian government means that his brazen violation of the U.N. Charter will be similarly overlooked. This is unfortunate for many reasons. Trampling on the U.N. Charter is harmful enough on its own, but it is even worse when that violation is celebrated for upholding an international norm. Norm-enforcement isn’t a good reason to violate the Charter, and even if the norm-enforcement “works” it comes at the high cost of breaking international law. Trump’s action also provides another precedent that later presidents can use for their own illegal wars in the future.
Predictably, many of the same people that worried that Trump would wreck the “international order” are thrilled that he is violating the U.N. Charter because he is doing it by attacking a government they have wanted to attack for years. Then again, many of the people that have been touting the virtues of “international order” have never cared whether the U.S. was adhering to the rules of that order, and that is on display once again for all to see.
There is no apparent justification for President Trump not to have asked Congress for such authorization here, and to have held off on the strikes until receiving such authorization. Therefore, this might turn out to be the rare case in which the President simultaneously violates both the Constitution and the Charter.