Which was the last US battalion to leave Vietnam?

On August 11, 1972, the last U.S. ground combat unit in South Vietnam, the 3rd Battalion, 21st Infantry, departed for the United States. The unit had been guarding the U.S. air base at Da Nang. This left only 43,500 advisors, airmen, and support troops left in-country. This number did not include the sailors of the Seventh Fleet on station in the South China Sea or the air force personnel in Thailand and Guam.

The 21st Infantry Regiment ("Gimlet") is a United States Army infantry regiment. The 1st and 3rd Battalions of the 21st currently exist as part of the 25th Infantry Division. The regiment fought in World War II, the Korean War and the Vietnam War, as well as Operation Iraqi Freedom. Task Force Smith, the first American unit to see action in the Korean War, was derived from the regiment's 1st Battalion.