First principles first. The Constitution expressly allocates to Congress a lengthy list of foreign affairs-related powers, not only to declare war, but also to regulate commerce with foreign nations, define and punish offenses against the law of nations, make rules for the government and regulation of the armed forces, appropriate funds to provide for the common defense, and indeed make “all laws which shall be necessary and proper for carrying into execution” any of those powers, among others.
The list of foreign-affairs powers the Constitution allocates to the president is, in contrast, quite brief and—far more important, as the Supreme Court has long and repeatedly explained—dependent “upon their disjunction or conjunction with those of Congress.” While the scope of some of the president’s powers may be reasonably broad, the president is on the thinnest possible constitutional ice when he takes steps “incompatible with” views Congress has expressly or impliedly made known. As Justice Robert H. Jackson famously explained in the Steel Seizure case during the Korean War, our constitutional equilibrium is one that ensures the president is bound by the duly enacted laws of the United States.
Far from understanding these powers to be more constrained in the realm of foreign affairs or national security, the Constitution’s Framers recognized the power to appropriate money in particular as an especially important check on the executive’s ability to exercise U.S. military power.