Trump’s justification:

“The President authorized that strike pursuant to his power under Article II of the Constitution as Commander in Chief and Chief Executive to use this sort of military force overseas to defend important U.S. national interests. The U.S. military action was directed against Syrian military targets directly connected to the April 4 [2017] chemical weapons attack in Idlib and was justified and legitimate as a measure to deter and prevent Syria’s illegal and unacceptable use of chemical weapons.”