The War Powers Resolution is a defensive exception to the rule:
The constitutional powers of the President as Commander-in-Chief to introduce United States Armed Forces into hostilities, or into situations where imminent involvement in hostilities is clearly indicated by the circumstances, are exercised only pursuant to (1) a declaration of war, (2) specific statutory authorization, or (3) a national emergency created by attack upon the United States, its territories or possessions, or its armed forces.
It wasn’t illegal, unfortunately. He has the power to do it for 60 days before Congress has to weigh in.
It’s illegal.
The War Powers Resolution is a defensive exception to the rule:
https://www.law.cornell.edu/uscode/text/50/1541