Presidential War Powers
Consider the following hypothetical question. If all members of Congress were killed by a missile attack launched from a foreign country, does the President have the power to legally activate the military against that country, or is he legally required to wait for Congress to be re-elected and declare war before he can act?
War Powers Resolution
The War Powers Resolution (US Code Title 50 Section 1541) grants the President the power to activate the military in the event of an attack on the United States.
(c) Presidential executive power as Commander-in-Chief; limitation
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.
The President can legally engage the US military in hostilities without a declaration of war from Congress in the event of an attack on the United States.