When the Great War began in Europe in August 1914, aviation in the U.S. Army was a function of the Signal Corps. Its mission was to support infantry and artillery by providing observation services. There were people in the Army and outside who could foresee no other military use for aircraft. Others believed that the airplane should be given a combat role. Some thought that aviation should be a separate combat arm. As news of aerial operations in the European war drifted across the Atlantic, there was more and more talk of buying fighting planes, battle planes, and bomb-dropping planes for the U.S. Army. Although some faltering steps were taken, little progress was made toward building a fighting air force-or even toward developing observation services for the ground forces-before the United States entered the war in April 1917. Having joined Great Britain, France, and Italy in war against Germany and Austria, the United States quickly drew plans for an air service to include fighting and bombing airplanes as well as observation planes and balloons. The allies, who had been fighting for two years or more and were much farther advanced in military aviation, provided valuable information to help the United States build up its air service. Allied contributions included not only technical data on new developments in aircraft and other aeronautical equipment but also information on how to use aviation in battle. One thing borrowed from the allies was the classification of military aviation into three, broad, functional areas labeled "observation," "pursuit," and "bombardment." Another was the concept that aviation had two separate roles, one "tactical," the other "strategical." In addition, the Allies provided ideas, along with detailed information about tactics, techniques, and procedures, that formed the foundation for the employment of the Air Service, American Expeditionary Forces (AEF), in World War I. The wartime operations of the Air Service, AEF, included visual and photographic reconnaissance, artillery adjustment, infantry liaison, counterair operations, bombing and strafing in close support of ground forces, and interdiction of the enemy's lines of communications. The war ended before US. units were ready for strategic air operations, but thinking on strategic air warfare had advanced far enough to raise, and tentatively answer, questions concerning area versus precision bombing and day versus night operations. Thus, US. military aviation in November 1918 was far different from what it had been when the war began four years earlier.