He did not have a congressional authorization but he did provide a letter, consistent with the War Powers Act.

WHITEHOUSE: Press Briefing

After the war, Congress created the War Powers Act in recognition that wars might commence before congressional approval could be given.

FORBES: What Happened To The American Declaration Of War?

And when he ran for election, is that what he meant when he said he supported Presidents abiding by the War Powers Act?

WHITEHOUSE: Press Briefing