Bellvm Americana
Revolutionary War (1775-1783)
*British Empire

War of 1812 (1812-1815)
*British Empire

Mexican War (1846-1848)
*Mexico

Civil War (1861-1865)
*Confederacy

Spanish-American War (1898)
*Spanish Empire

Great War (1914-1918, US 1917-1918)
*German Empire
*Austro-Hungarian Empire
*Ottoman Empire

Second World War (1939-1945, US 1941-1945)
*Nazi Germany
*Japanese Empire
*Fascist Italy

Korean War (1950-1953)
*North Korea
*China
*QT: Soviet Union

Vietnam War (1957-1975, US 1965-1973)
*North Vietnam
*QT: China, Soviet Union

Gulf War (1991)
*Iraq

Afghan War (2001)
*Taliban militia
*al Qaeda

Iraq War (2003-present)
*Iraq
*al Qaeda

An argument can be made for not including the Gulf War. After all, it only lasted a few weeks (and the ground war only a few days), and was entirely one-sided. But, the Spanish-American War can't reasonably be left off the list ("Remember the Maine!", San Juan Hill, America's first and only overseas empire) even it lasted only a few months and was devastatingly one-sided. So, the 1991 Gulf War is in.