The Army's primary use for the S-58 was as a troop and supply transport; the first Army CH-34 (S-58) flew in March 1955. In 1957 Sikorsky Aircraft developed an armament system for the CH-34, which gave even greater combat and strike capability to the helicopter. By 1958 the Army's CH-34s were on duty throughout the United States, Europe, and Asia. The first models entered service with the U.S. Army in April 1955. By 1958, the CH-34 was the principal U.S. Army transport helicopter. The CH-34A had a crew of 2 plus 16 to 18 troops. It had no armament. The CH-34A was equipped to carry external sling loads, and it had automatic stabilization equipment. The helicopter could carry up to 8 stretchers and a medic. The U.S. Army had been issued 437 CH-34A/B/C's by 1970. South Vietnam had, on average, about 62 CH-34 helicopters. The Choctaw was first acquired by the South Vietnam Air Force in 1962 and, by the end of 1968, it had five squadrons of CH-34 helicopters. By the end of 1970, only 23 remained in service in the Vietnam Air Force.
The U. S. Army employed the H-34 principally for general utility purposes, as well as VIP transport flights, and SAR missions. One of the most challenging missions flown by Army H-34s was the evacuation of the Congo in 1964, but Army H-34s did not participate in Vietnam, and did not fly in the assault helicopter role.
Pilots of H-34s flying in Vietnam discovered in the combat zone that some of the design's innovative features carried penalties. The high cockpit made it an obvious target, and the drive shaft created a partition that made it difficult for crew chiefs to come to the aid of the cockpit crew if they became injured. The H-34's magnesium skin resulted in very intense fires, and contributed to significant corrosion problems. The airframe was also too weak to support most of the weapon systems that allowed the UH-1 to become an effective ad-hoc gunship. Nonetheless, the H-34 demonstrated an ability to sustain a substantial amount of combat damage and still return home.
Early in 1965, Operation SHUFLY ended as U. S. Marine and Army units landed in Vietnam, following the Tonkin Gulf resolution, and took the lead in the war against the Viet Cong. In March 1966, the more capable turbine-powered CH-46A began to replace the UH-34s. However, in August 1967, several fatal crashes caused by tail pylon failures resulted in the grounding of the CH-46As, and the somewhat haggard but reliable H-34 remained in service until engineers resolved the CH-46 structural problems.
In the late 1950s, Air America, a CIA-created airline, began flying UH-34Ds in Laos, manned by crews on leave from the Marine Corps. When the last military UH-34 left Vietnam, Air America was still in operation with the type, including upgraded S-58Ts powered by the powerful turbine PT6T-6 "TwinPac."