Instances of Use of United States Armed Forces Abroad: 1798-2004 [October 5, 2004] [open pdf - 157KB]
From the Summary: "The instances differ greatly in number of forces, purpose, extent of hostilities, and legal authorization. Eleven times in its history the U.S. has formally declared war against foreign nations. These eleven U.S. war declarations encompassed five separate wars: the war with Great Britain declared in 1812, the war with Mexico declared in 1846, the War with Spain declared in 1898, the First World War, during which the U.S. declared war with Germany and with Austria-Hungary during 1917, World War II, during which the U.S. declared war against Japan, Germany, and Italy in 1941, and against Bulgaria, Hungary, and Rumania in 1942. Some of the instances were extended military engagements that might be considered undeclared wars. These include the Undeclared Naval War with France from 1798 to 1800; the First Barbary War from 1801 to 1805; the Second Barbary War of 1815; the Korean War of 1950-53; the Vietnam War from 1964 to 1973; the Persian Gulf War of 1991, global actions against foreign terrorists after the September 11, 2001 attacks on the United States, and the War with Iraq in 2003. With the exception of the Korean War, all of these conflicts received Congressional authorization in some form short of a formal declaration of war. Other, more recent instances often involve deployment of U.S. military forces as part of a multinational operation associated with NATO or the United Nations. Current legislation, H.R. 10 and S. 2845, contains provisions that could strengthen the intelligence capabilities of the United States to combat international terrorism and to support current and future American military operations overseas in a more effective manner."
CRS Report for Congress, RL32170