II Field Force Vietnam arrived in the Republic of Vietnam March 15, 1966, and at its height became one of the largest corps-level commands in the history of the U.S. Army. II Field Force traces its lineage to the XXII U.S. Army Corps formed in 1944 in the European campaign. Inactivated at the end of World War II, it was reactivated at Fort Hood, Texas with the coming of the Vietnam war and renamed II Field Force.

   Their area of responsibility was III Corps Tactical Zone also known as Military Region 3, the 11 provinces surrounding Saigon. At various times it had under its command the following major units of elements:1st, 9th and 25th Infantry Divisions; 101st and 82nd Airborne Divisions; 173rd Airborne Brigade; 1st Cavalry Division (Airmobile), 11th Armored Cavalry Regiment, 12th Combat Aviation Group, 23rd and 54th Artillery Groups, and the 1st Australian Task Force and Royal Thai Army Volunteer Force.

   II Field Force units were responsible for the decisive defeat of enemy forces in Military Region 3 during the 1968 Tet offensive, which threatened particularly the Saigon area. But its greatest moment was the Cambodian incursion of May and June 1970 when the 25th Infantry Division, 1st Cavalry and 11th Armored Cavalry fought alongside ARVN troops of III Corps in one of the war's biggest and most successful operations.