The Korean War ends in 1953, 11 years before the Gulf of Tonkin incident (1964) and US cranking up its involvement in Vietnam.
WWII ends in 1945, 5 years before US involvement in Korea.
US involvement in WWII began in 1941, 23 years after the 1918 end of WWI (the war to end all wars). The US got involved in WWI in 1917, 19 years after it began and ended the Spanish American War in 1898.
The Spanish American War came 33 years after then end of the Civil War (1865), a fairly large break in the action if you don't count the various Indian/ US conflicts during that time (which I'm not).
The Civil War starts in 1861, 13 years after we got done fighting with Mexico (1848). That war starts in 1846, 32 years after we made peace with the British for the second time (1814). Of course, our first go around with the British ended in 1783, 29 years before the second fight.
Anyway, average that out and you find that as a country every 18 years or so we're at war. FYI, this isn't intended to be some sort of passive aggressive commentary on our country or our history. From what I know of history, it seems that most of the times we go to war its for the right reasons. I just found the 18 year number to be interesting.