We've been at war so long, it sometimes feels like it's always been this way. To help put this in perspective, I made this timeline of the years the US has been involved in an armed conflict. You might have to click it to make it large enough to read.
Most of the information comes from this article. I left a few off, somewhat arbitrarily. I didn't include seven minor conflicts with China (years 1918, 1921, 1926-1927, 1930, 1937, 1945-1947, and 1956) because they didn't have a name, and I left off American Expeditionary Force Siberia because it seemed a lot like the Northern Russian Expedition, which occurred in the same years. (Also left out: the Polar Bear Expedition.) I included two that are kind of questionable. The Mexican Revolution, in which 35 US troops were killed and 70 injured, as well as the El Salvador Civil War, in which 20 were killed and 35 wounded. While we were certainly entangled in both these events, the US efforts were primarily behind-the-scenes. The article I used listed conflicts were US soldiers were killed or wounded, and there are certainly interventions in which there were no US casualties, so that is another shortcoming of this timeline.
I have since noticed an error: I have the Quasi-War starting in 1778, when in fact it started in 1798. My sincerest apologies to any time travelers who went to 1778 in an attempt to stop the Quasi-War. Hopefully you enjoyed the June 24th solar eclipse or perhaps the Battle of Monmouth during your stay in 1778.
The timeline above is close to, but does not exactly match, what I pictured. This one is closer. Does anyone know enough about graphs to figure out how to do something kind of like a combination of the two?
I fixed a few things, and included a few more instances, based largely on this article. It really is arbitrary what you choose to include. I tried not to show every anti-pirate action, every foreign deployment, and every clash with a Native American tribe, but whether something is significant or not is just a judgment call.