Here is a chart I put together of instances of U.S. use of military force abroad, 1800-2009, by decade. Two caveats: 1) the numbers by themselves say nothing about the severity of any conflict, and 2) some cases are double-counted because they cross decades.
Source: Richard F. Grimmett, Instances of Use of United States Armed Forces Abroad, 1798-2010. Congressional Research Service. March 10, 2011.