Air mobility has been a key concept in offensive operations since the 1930s. Initial approaches to air mobility focused on airborne and glider-borne troops. During World War II many assaults were done by military gliders. The World War Two era German Fallschirmjäger, Brandenburgers, and the 22nd Air Landing Division glider borne paras laid the foundation for modern day air assault operations. In 1941 the U.S. Army quickly adopted this concept of offensive operations initially utilizing wooden gliders before the development of helicopters. Following the war, faster aircraft led to the abandonment of the flimsy wooden gliders with the then new helicopters taking their place. Four YR-4B helicopters saw limited service in the China Burma India theatre with the 1st Air Commando Group