According to the U.S. Constitution, only Congress has the power to declare war. However, often a U.S. president has initiated armed conflict and requested congressional approval afterwards. Also, in lieu of a formal declaration of war, Congress frequently authorizes the use of military force.
Article one, section eight, clause 11 of the Constitution states that "The Congress shall have power to declare war." Clauses 12 to 14 add that Congress has power to raise and support armies, provide and maintain a navy, and regulate the land and naval forces. Another clause designates the president as the commander-in-chief of the Army and Navy.
As of 2014, Congress has formally declared war 11 times. The first occasion was a declaration of war on Great Britain in the War of 1812, and the last were declarations of war on Japan, Germany, Italy, Bulgaria, Hungary and Romania during World War II. Since then, the United States has justified warfare by Congressional authorization of the use of military forces, as in the Vietnam War, the second Persian Gulf War, the Afghanistan War and the Iraq War, or by Congressional funding of forces deployed after U.N. Security Council resolutions, as in the Korean War, the first Persian Gulf War and the Libyan Civil War. On numerous occasions, U.S. presidents have instituted military action without any Congressional approval, as in the Philippine-American War from 1898 to 1903 and the 1999 NATO bombing of Yugoslavia.