On March 19, 2008, the world noted the 5th anniversary of the invasion of Iraq. The Iraq War is now the third longest war in American history, after the Vietnam War and the continuing war in Afghanistan.
Below is a look at America’s major wars and their length in months. Time periods are rounded up or down for ease of comparison. Current conflicts are italicized and are colored red. The longest wars are listed first in descending order by length. The start dates reflect when the United States entered the wars.
Vietnam War–August, 1964 to April, 1975= 129 months (American involvement began in the late 1950s, but major U.S. combat forces began taking part in large-unit combat in 1964. August, 1964 is the month of the Gulf of Tonkin incident and the U.S. bombing of North Vietnamese targets.)
American Revolution–April, 1775 to September, 1783= 100 months
Afghanistan– October, 2001 to Present (as of March, 2008)= 78 months
Iraq War–March, 2003 to Present (as of March, 2008)= 60 months
U.S. Civil War–April, 1861 to April, 1865= 48 months
World War II–December, 1941 to September, 1945= 45 months
World War I–April, 1917 to November, 1918= 19 months
Korean War– June, 1950 to July, 1953= 37 months
War of 1812–June, 1812 to February, 1815= 32 months
U.S.-Mexican War– May, 1846 to February, 1848= 21 months
Spanish-American War–April, 1898 to August, 1898= 5 months
Gulf War–January, 1991 to March, 1991= 3 months
Source: http://www.historyguy.com/american_wars_by_length_of_time.html