"No power but Congress can declare war; but what is the value of this constitutional provision, if the President of his own authority may make such military movements as must bring on war? ... [T]hese remarks originate purely in a desire to maintain the powers of government as they are established by the Constitution between the different departments, and hope that, whether we have conquests or no conquests, war or no war, peace or no peace, we shall yet preserve, in its integrity and strength, the Constitution of the United States."