President Obama’s aborted threat to intervene in Syria in 2013 has led many to forgot that the administration believed it had the domestic constitutional authority to intervene without congressional authorization. Even when President Obama announced that he would seek congressional authorization for the strike, he insisted that “I have the authority to carry out this military action without specific congressional authorization.”... As Charlie Savage reported [in the NYT in 2013]:
In recent weeks, administration lawyers decided that it was within Mr. Obama’s constitutional authority to carry out a strike on Syria as well, even without permission from Congress or the Security Council, because of the “important national interests” of limiting regional instability and of enforcing the norm against using chemical weapons, as [Kathryn Ruemmler, the White House counsel,] said.
April 7, 2017
"I can imagine the smile on Trump administration officials’ faces when they figured out that they would both enforce a red line that Obama wouldn’t and rely on Obama administration legal thinking to provide cover for doing so."
Writes lawprof Jack Goldsmith in "The Constitutionality of the Syria Strike Through the Eyes of OLC (and the Obama Administration)."