In our short history the United States has had it's fair share of wars and military involvement . I like to think and I truly believe that most of them where justified and some even needed for the survival of the USA and democracy in general. On the other hand there have been a few that aren't all that justified.

The American Revolution 1775 to 1783
Most everyone knows the basics of the Revolutionary War. We declared independence from Great Brittan passed by congress on July 2nd 1776 and adopted on July 4th. The underlying cause of the war was Brittan's refusal to treat the American colonies as equal British citizens including the fact we had no voice in Parliament.
Franco-American War 1798 to 1800
The Barbary Wars 1801 to 1805 and 1815
War of 1812 1812 to 1815
Creek War 1813 to 1814
War of Texas Independence 1836
The Mexican War 1846 to 1848
The American Civil War 1861 to 1865
Spanish American War 1898
WWI 1914 to 1918 (US participation 1917 to 1918)
WWII 1939 to 1945 (US participation 1941 to 1945)
Korean War 1950 to 1953
Vietnam War 1960 to 1975
Bay of Pigs 1961
Grenada 1983
Panama 1989
Gulf War (Desert Storm) 1990 to 1991
Bosnia 1995 to 1996
War on Terror (Enduring Freedom) 2001 ongoing
Iraq Freedom 2003 ongoing