From the Council on Foreign Relations: U.S. Foreign Policy Powers: Congress and the President

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The U.S. Constitution parcels out foreign relations powers to both the executive and legislative branches. It grants some powers, like command of the military, exclusively to the president and others, like the regulation of foreign commerce, to Congress, while still others it divides among the two or simply does not assign.

The separation of powers has spawned a great deal of debate over the roles of the president and Congress in foreign affairs, as well as over the limits on their respective authorities. “The Constitution, considered only for its affirmative grants of power capable of affecting the issue, is an invitation to struggle for the privilege of directing American foreign policy,” wrote constitutional scholar Edward S. Corwin in 1958.

Foreign policy experts say that presidents have accumulated power at the expense of Congress in recent years as part of a pattern in which, during times of war or national emergency, the executive branch tends to eclipse the legislature.

The periodic tug-of-war between the president and Congress over foreign policy is not a by-product of the Constitution, but rather, one of its core aims. The drafters distributed political power and imposed checks and balances to ward off monarchical tyranny embodied by Britain’s King George III. They also sought to remedy the failings of the Articles of Confederation, the national charter adopted in 1777, which many regarded as a form of legislative tyranny. “If there is a principle in our Constitution, indeed in any free Constitution, more sacred than any other, it is that which separates the legislative, executive, and judicial powers,” wrote James Madison, U.S. representative from Virginia, in the Federalist papers.