This seminal casebook helped pioneer the field of U.S. foreign relations law. The 5th edition provides comprehensive treatment of the distribution of foreign relations powers within the federal government; customary international law and treaties as sources of law in the United States; the war powers; and Congress's power of the purse, federalism, and the justiciability of foreign relations cases. Particular emphasis is placed upon historical sources, materials which document the practices of the executive branch, and key doctrines of international law. Practice exercises are provided to stimulate student engagement.