When can president use armed forces?
It depends on exactly what you mean by "use the military". The President is Commander-in-Chief of the US Armed Forces, and "uses" the military fairly frequently. The Presidential helicopter, for example, is "Marine One", piloted and staffed by the USMC. The Presidential airplane is "Air Force One", piloted and staffed by the USAF. (It should be noted that there is no plane officially named "Air Force One"; that designation is given to whatever US Air Force aircraft the President happens to be on at the time. Usually it's one of two VC-25s -- modified 747s -- with tail numbers 28000 and 29000).Officially, the power to actually declare war lies with Congress, but the last time Congress actually did so was in 1941. Every military action since then has not ... technically ... been a "war". To allow for immediate response to urgent threats, the President can authorize military action without waiting for Congress to officially declare war. In some cases (for example, every one since World War II), Congress never winds up officially declaring war ... they just approve the President's actions and allow them to continue (theoretically they could instead repudiate the President's actions and order cessation of hostilities, but this hasn't ever happened from Congress as a body, though individual members have occasionally been fairly vocal about it).