Foreign relations under the US Constitution starts with the paradox, also seen in domestic matters, of relatively scant text providing guidance for the exercise of vast power. Founding understandings, structural inference, and ongoing constitutional custom and precedent have filled in much, though hardly all, of the framework over the course of two hundred years. As a result, two basic questions frame the relationship between the Constitution and US foreign policy: (1) which parts of the US government, alone or in combination, properly exercise authority in the making of foreign policy; and (2) once made, what is the status of the nation’s international legal obligations in the US domestic legal system.
The making of American foreign policy is framed by the Constitution’s commitment to separation of powers. Congress, the president, and the courts are all allocated discrete yet significant foreign affairs authority. Determining the exact borders and overlaps in areas such as the use of military force, emergency measures, and treaty termination continues to generate controversy. The status of international law in the US legal system in the first instance turns on whether resulting obligations derive from agreements or custom. The United States enters into international agreements in three ways: treaties, congressional-executive agreements, and sole executive agreements. Complex doctrine deals with the domestic applicability of treaties in particular. US courts primarily apply customary international law in two basic ways. They can exercise a version of their common lawmaking authority to fashion rules of decision based on international custom. They also apply customary international law when incorporated into domestic law by statute.