Airmobile operations involved the use of helicopters to transport troops into battle and to provide fire support at battle sites. Instead of transporting troops into battle while artillery barrages prepared the way, airmobile operations had transport helicopters move troops simultaneously with artillery and gunship fire, keeping the enemy off guard. During the Vietnam War, airmobile operations were used extensively by the United States Army and Marine Corps, but the major airmobile unit was the 101st Airborne Division. Since airmobile operations had not even been tested until the 1950s when reliable helicopters were functioning, Vietnam became the combat breakthrough for airmobile tactics. They proved to be the foremost tactical innovation of the Vietnam War.