The following is the Dec. 28, 2018 Congressional Research Service report, Instances of Use of United States Armed Forces Abroad, 1798-2018.
From the Report
This report lists hundreds of instances in which the United States has used its Armed Forces abroad in situations of military conflict or potential conflict or for other than normal peacetime purposes. It was compiled in part from various older lists and is intended primarily to provide a rough survey of past U.S. military ventures abroad, without reference to the magnitude of the given instance noted. The listing often contains references, especially from 1980 forward, to continuing military deployments, especially U.S. military participation in multinational operations associated with NATO or the United Nations. Most of these post-1980 instances are summaries based on presidential reports to Congress related to the War Powers Resolution. A comprehensive commentary regarding any of the instances listed is not undertaken here.
The instances differ greatly in number of forces, purpose, extent of hostilities, and legal authorization. Eleven times in its history, the United States has formally declared war against foreign nations. These 11 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 United States declared war with Germany and with Austria-Hungary during 1917; and World War II, during which the United States 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 1950-1953; 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 have often involved deployment of U.S. military forces as part of a multinational operation associated with NATO or the United Nations.
The majority of the instances listed prior to World War II were brief Marine Corps or Navy actions to protect U.S. citizens or promote U.S. interests. A number were engagements against pirates or bandits. Covert operations, domestic disaster relief, and routine alliance stationing and training exercises are not included here, nor are the Civil and Revolutionary Wars and the continual use of U.S. military units in the exploration, settlement, and pacification of the western part of the United States.
Download the report here.