In the United States, the Aviation Section was nearly overwhelmed with the problems of rapid expansion to fight a modern war—the recruitment and training of pilots and mechanics, the production of airplanes, the formation and equipping of combat units, and the acquisition of air bases—while overseas a second force developed as part of the American Expeditionary Force, absorbing most of the experienced leadership of military aviation and taking over much of the expansion responsibilities except aircraft production. This second force, the Air Service of the AEF, used European-built aircraft and training facilities and forced the separation of aviation from the Signal Corps.