The constitutionality has never been officially settled, but it's another form of manipulating the Constitution and the fact that, like it or not, politics play a part in everything. Technically speaking, it might be unconstitutional, but if it is it's because of an unconstitutional decision made by Congress in the early 70s, which wasn't unusual for the time. United States involvement in Korea and Vietnam were "police actions", or war without it being declared. Particularly after Vietnam, some people looked to stop the president from using this end around for using military force, so congress passed the War Powers Resolution in 1973 which requires the president to get approval of Congress 60 days before using military force. The fact of the matter is this is no different than a declaration of war. It's just a way of someone having the room to technically say, "Hey, this was his idea."