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From Americans at War in Foreign Forces:
The German invasion of Belgium and declaration of war against France on August 3, 1914 was the beginning of a sometimes romantic, though violent, engagement of Americans in the struggle of the invaded against the Germans. But it was England's declaration of war against Germany on the following day that began the first footsteps of what would become a virtual migration of Americans into British forces over the next three years. It began with the largely unobserved travel of individual Americans across the Canadian border and into Commonwealth Forces.
As in France, however, Americans in the sky had more visibility. By the time the United States entered the war in 1917, an estimated 300 Americans had joined, or were yet to join, England’s Royal Flying Corps (RFC) or Royal Naval Air Service (RNAS). The Royal Flying Corps had been created in 1912 as an army and navy effort to explore the potential use of aircraft in war. It eventually split off a second organization, the Royal Naval Air Service. Starting with the German invasion in 1914, the RFC struggled to keep up with needed aircraft and pilots until it attained 27 squadrons and at least 400 usable aircraft by July 1916. It continued to expand as the war progressed, and its needs for men and materiel were always pressing. By the end of the war, the RFC and RNAS had grown to a force of 290,000 personnel and approximately 22,000 aircraft. Their work on all fronts of the war would be considered decisive in the ultimate success of the allies. Drawn into the war by the same mix of adventurousness and/or sense of duty as their counterparts in French aviation, most American recruits in the RFC were trained in Canada before being sent to England for more training, and then to the front. Ten percent of the group was able to reach the status of “Ace” which was variously defined as having attained five or more kills.
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