U.S. Army CH-21 helicopter transports arrived in South Vietnam on 11 December 1961. Air assault operations using Army of the Republic of Vietnam (ARVN) troops began 12 days later in Operation Chopper. These were very successful at first but the Viet Cong (VC) began developing counter helicopter techniques, and at the Battle of Ap Bac in January 1963, 13 of 15 helicopters were hit and four shot down. The Army began adding machine guns and rockets to their smaller helicopters and developed the first purpose built gunship with the M-6E3 armament system.