Nothing in our Constitution is plainer than that declaration of a war is entrusted only to Congress. Of course, a state of war may in fact exist without a formal declaration. But no doctrine that the Court could promulgate would seem to me more sinister and alarming than that a President whose conduct of foreign affairs is so largely uncontrolled, and often even is unknown, can vastly enlarge his mastery over the internal affairs of the country by his own commitment of the Nation's armed forces to some foreign venture.

Youngstown Sheet & Tube Co. v. Sawyer, 343 U.S. 579 (1952) (concurring) - Judicial opinions