Now, in one sense, no doubt this is war, and may be a war of the most extensive and threatening dimensions and effects, but it is a statement simply of its existence in a material sense, and has no relevancy or weight when the question is what constitutes war in a legal sense, in the sense of the law of nations, and of the Constitution of the United States. For it must be a war in this sense to attach to it all the consequences that belong to belligerent rights. Instead, therefore, of inquiring after armies and navies, and victories lost and won . . . the inquiry should be into the law of nations and into the . . . fundamental laws of the Government. For we find there that to constitute a . . . war in the sense in which we are speaking, before it can exist, in contemplation of law, it must be recognized or declared by . . . the Congress of the United States. . . [W]ar, therefore, under our system of government, can exist only by an act of Congress, which requires the assent of two of the great departments of the Government, the Executive and Legislative.
Those words were written by Supreme Court Justice Samuel Nelson, in a time of great national crisis, to defend the Constitution againt an overreaching President who had called out troops without explicit Congressional authorization.
Before anyone gets too eager to apply Nelson's ringing phrases to current events, let me add a few details. The President in question was Abraham Lincoln, the war was the Civil War, and in the case at hand, Nelson was arguing that the blockade of Southern ports Lincoln imposed in the weeks after Fort Sumter was unconstitutional.
Ideas have a funny way of jumping ideological fences, don't they?