主题：九一一日记 （一） -- 穿越

It depends a lot on how you define war. In terms of a declared war, the American revolution was the longest, at 8 years. There were US combat forces engaged in Vietnam for somewhat longer (like 1959 to 1975), but the bulk of the US open engagement was just eight years, and additionally, it was not a declared war, but was instead a "police action." Iraq has stretched on for near 6 years now, and Afghanistan has gone on for more than seven years now.

If you include Vietnam, then why not the Cold War, which ran from 1945 through 1990.