he needed congressional approval to act. he failed to get it or even ask for it. that's unconstitutional.
at least he warned putin and assad ahead of time. brilliant.
Ehhh Politifact examined this and came down on the other side of it.
Mark Pocan wrongly claims Donald Trump had no legal authority to launch missile attack on Syria
"[...]
There’s certainly debate over the extent of a president’s authority to use military force without approval from Congress.
But Pocan went too far in saying there is no legal basis for Trump’s action.
[...]
Experts agree that in limited instances, such as the Syrian missile attack, a president has legal authority provided in the Constitution as commander-in chief.
[...]
"While the precise scope of this power is unclear, a strong argument can be made that the president can use force in short military operations -- especially where there is minimal risk to American lives -- without congressional authorization. Indeed, over the years, Congress has generally acquiesced in such presidential uses of force.
"Because the air strikes were undertaken by cruise missiles that put virtually no American lives at risk and because the strikes lasted only minutes, the president's action would seem to be a lawful use of force under the Constitution. Needless to say, if further military actions were to be undertaken, they could rise to the level of requiring congressional authorization."
[...]
- Since the last time Congress declared war, at the beginning of World War II, presidents have generally initiated military activities using their constitutionally granted powers as commander in chief without having an official declaration of war in support of their actions.
- Even under the War Powers Resolution, the president can send in forces without approval from Congress.
- Lower courts have ruled in favor of the White House in the use of force, and the U.S. Supreme Court declined to hear an appeal on that point.
[...]
Pocan said: "There is no legal basis" for Trump's "missile strike against Syrian military assets."
For limited military activities like the missile strike, presidents can send in forces without approval from Congress.
We rate Pocan’s statement False."
http://www.politifact.com/wisconsin/statements/2017/apr/07/mark-pocan/mark-pocan-wrongly-claims-donald-trump-had-no-lega/