The U.S. Army first employed aircraft operationally in the Punitive Expedition against Pancho Villa in 1916-1917, gaining valuable experience in the logistics and tactics of air operations. Largely in response to Germany's unrestricted submarine warfare, the United States declared war on Germany on 6 April 1917. Entry into this global conflict triggered a massive mobilization to recruit, equip, and train U.S. forces. The U.S. Army Air Service incorporated both doctrine and aircraft from British, French and Italian air services, taking advantage of their years of combat experience. It grew into a versatile and effective force during the conflict, operating primarily in support of U.S. and allied ground forces. In this first major use of aerial forces, the United States executed many missions that still characterize air warfare today. Airmen flew reconnaissance missions to locate enemy forces, while pursuit aircraft engaged in aerial dogfights to prevent the other side from gathering information on the location and status of U.S. troops. The Air Service also flew infantry contact patrols in an attempt to locate both friendly and enemy units. Airmen risked their lives in low-level close air support missions, and bombers conducted attacks on enemy forces and infrastructure. Fighter pilots like Eddie Rickenbacker, the leading American ace in the war with 26 aerial victories, and Frank Luke caught the imagination of the American public. Technology advanced rapidly under the pressure of combat, and by 1918 Americans possessed aircraft and techniques specializing in a variety of aerial missions. The Air Service made significant contributions during the St. Mihiel Offensive and the Meuse-Argonne Offensive in autumn 1918, supporting U.S. ground forces in the offensives that would bring the war to a close. In seven months of combat, the United States Air Service logged 150 bombing attacks and claimed 781 enemy aircraft and 73 balloons, confirming airpower's growing role in modern American warfare. During the war, the Army Air Service grew from 1,218 personnel in 1917 to 195,023 in 1918.