It goes back to the War Powers Resolution
The War Powers Resolution requires the President to notify Congress within 48 hours of committing armed forces to military action and forbids armed forces from remaining for more than 60 days, with a further 30 day withdrawal period, without an authorization of the use of military force or a declaration of war. The resolution was passed by two-thirds of Congress, overriding a presidential veto. It has been alleged that the War Powers Resolution has been violated in the past, for example, by President Clinton in 1999, during the bombing campaign in Kosovo. All incidents have had congressional disapproval, but none have had any successful legal actions taken against the president for alleged violations.[2]
Presidents don't like even that restriction, but there's enough wiggle-room in it that they pay lip-service to it.
FWIW.
Cheers,
Scott.