The Constitution of the United States grants the power to declare war to Congress. Article I, Section 8 states that "Congress shall have power to ... declare war, grant letters of marque and reprisal, and make rules concerning captures on land and water."
The president is the commander-in-chief of the armed forces and commands military operations after a declaration of war. However, presidents frequently order military operations without an explicit declaration of war by Congress. The War Powers Resolution of 1973 is intended to address such situations by requiring the president to notify Congress within 48 hours of committing troops. Troops must be withdrawn within 60 days unless Congress allows an extension.