The 1st Cavalry Division traces its roots back to the 2nd Cavalry Regiment, formed in 1855 and later redesignated 5th Cavalry. The 5th, 7th, 8th and 9th Cavalry Regiments from the mid-1800's are still in existence today, as part of the Division. (This from the official 1st Cav site; Wikipedia makes the origins as 1st Cavalry Regiment in 1833, but that regiment is no longer represented.) There were several entities called Cavalry Divisions during the Civil War, but they were not numbered. What we know as the 1st Cavalry Division was formed in 1921 at Fort Bliss, Texas, and were used through the 1920's and 30's to patrol the desert along the Mexican border. In World War II they were converted to infantry. In Vietnam they were converted to an air assault division using helicopters. (We in the 1st Aviation Brigade always thought they were a little full of themselves, and I still feel some jealousy that it's always them featured in the movies, but I guess they were ok.) In 1975 the 1st Cavalry became a heavy armored division. Current home base is Fort Hood, Texas.
Several regiments participated in the Civil War and Indian Wars, though the 1st Division did not exist yet.
World War II (Philippines)
Korea
Vietnam
First Gulf War
Second Gulf War