This list counts major wars until 2016. Some of the wars led in this time were of defensive nature. This list does not account for covert operations and other, similar acts that might be considered war.
Since 1955: The Korean war was never ended, and there was an active and continued military engagement with daily exchanges in Iraq from 1991 through 2003.
The mainland United States has not been attacked as a part of a military operation in the modern era, so the defensive claim is questionable.