Iraq war, this is a counterstamped nickel I found in a roll search ca. 2006. Obviously someone had strong views on the current situation and thought they'd get their message out by counterstamping circulating coinage.
I'll proffer 1995 - when the U.S. (along with NATO) was involved with the Former Republics of Yugoslavia - Serbia, Croatia, Bosnia, Kosovo, Slovenia, Macedonia . . . Did I get them all or add too many?
Unfortunately I bet you could pick almost any year and the US would have been fighting somewhere... whether it was with Indian tribes well up into the 19th century, or the banana wars in the early 20th century, or unacknowledged fighting almost everywhere since 1945. My ex father in-law, who was USAF, was stationed in Thailand in the very early 1960s (to help with the bombing of the Ho Chi Minh trail in Laos), when the US Government went ON RECORD as saying there were no US forces in Thailand. He and his wife, when they came back to the US and he was honorably released from service, got a lot of crap from civilians when they said that the US Government was lying. Needless to say, they were later proven right. To the Mother of All Wars that was, Thank God, missed...