I believe the President can order Marines into an area to not exceed 30 days before he as to go to congress and declare war. Military action though doesn't mean a declaration of war. A lot of spec ops fly under the black flag without anyone, but the executive branch knowing. Yeah, I'm not too keen with the executive branch ordering big military operations without congress' approval.
The original intention of that was in terms of a relatiatory strike, or an effort to prevent an offensive against the US. However, like most things, it has been mutated and used without discretion in nearly all cases.