Wikipedia has a page showing all (or at least many) of America’s military engagements at home and abroad in two timelines, 1770 to 1900 and 1900 to present (h/t Juan Cole).
The first thing you might note is that we have gone to war a lot. Most of the 19th-century campaigns were against this or that Indian nation—manifest destiny stuff, mostly—but there are also lesser-known foreign engagements like the Philippine-American War (1898-1902), which resulted in more than 4,000 American deaths, mostly from disease, and as many as 1.5 million Filipino civilian deaths. We also seemed to like to do some occupying back in the day, including Nicaragua (1912-33), Haiti (1915-34), the Dominican Republic (i.e. the other half of the island with Haiti) (1916-24), and the Dominican Republic again (1965-66).
The timelines color-code each war or conflict to indicate whether the conflict is ongoing or whether the U.S. was the winner, loser, or neither. It identifies seven “ongoing” conflicts: Continue reading

It would appear, at least according to some researchers, that 

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The short answer is “never,” but bear with me.![By U.S. Air Force photo by Tech. Sgt. Keith Brown [Public domain], via Wikimedia Commons By U.S. Air Force photo by Tech. Sgt. Keith Brown [Public domain], via Wikimedia Commons](http://crypticphilosopher.com/wp-content/uploads/2014/02/800px-F-15_pilots_Elmendorf.jpg)


![By Richard Bartz (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons](http://crypticphilosopher.com/wp-content/uploads/2014/02/VW_Werk_Altes_Heizkraftwerk-300x145.jpg)
