The Constitution expressly and exclusively vests in the Legislature the power of declaring a state of war [and] the power of raising armies.... A delegation of such powers [to the President] would have struck, not only at the fabric of our Constitution, but at the foundation of all well organized and well checked governments. The separation of the power of declaring war from that of conducting it, is wisely contrived to exclude the danger of its being declared for the sake of its being conducted.