https://constitution.congress.gov/browse/essay/artI-S8-C11-2...

Use of military force requires congressional approval.

Well, in principle. In practice, the US executive does not observe this restriction, or at most - makes a flimsy connection the 2001 AUMF following the twin towers attack. The courts do not enjoin it from using military force pretty much arbitraly; and congress does not impeach nor even adopt declarative denunciations of this behavior.

George Washington was the first president to take military action without congressional approval, so on the sense of precedent providing legality, it's quite an old concept.