Except (among others):
1) The US intervention in the insurgency in Cuba (the "Sugar Intervention") which ran from 1917-1922.
2) The US intervention in China from about 1922-1927.
3) The use of the US military to clear the protesting "Bonus Army" of US veterans in Washington D.C. in 1932.
There may be 21 years without armed conflict by the US in its history, but they aren't 21 years in a row.
The original commenter is trying to compare the number of years the US was involved in military conflicts vs. Europe. If you're going to include events as obscure as the sugar intervention (of which I found very little mention in books about Cuban history and US/Caribbean relations) then I doubt the comparison would be very meaningful.
The conflicts that became WWII started at different times in different places.