Yanks in British Commonwealth Forces in the Great War

[Morris at Heathfield]

Before the United States entered the war, Americans who took part in the war (on the Allied side) tended to join the French Foreign Legion, the Escadrille Lafayette, or other French squadrons. However, some did fight in the British forces during the war. One of the Escadrille Lafayette pilots, James Norman Hall (later author of Mutiny on the Bounty), pretended to be a Canadian and served as a machine gunner in the Royal Fusiliers before his real nationality was discovered and he returned to the U.S. until the formation of the Escadrille Lafayette. He wrote a book about his experience in the Royal Fusiliers, titled Kitchener's Mob.

Oliver Colin LeBoutillier, who was born in New Jersey to an English father and a Canadian mother, served in No. 9 Squadron of the Royal Naval Air Service, later 209 Squadron of the RAF. He was involved in the air battle in which Manfred von Richthofen (the Red Baron) lost his life, and was one of the few who witnessed the fight and subsequent crash from the air. LeBoutillier was credited with ten aerial victories. After the war he became a barnstormer, skywriter, and stunt pilot, and worked on such movies as Wings and Hell's Angels.

Then there's Frederick Libby, a cowboy from Colorado, living in Canada at the time war broke out, who joined the Canadian Expeditionary Force and was assigned to motor transport duty supporting the Canadian 2nd Division. He later volunteered to join the Royal Flying Corps, and served as an observer in a two-seater aircraft. In this capacity he became the first American ace (less than two months before the French-American Escadrille Lafayette fighter pilot, Raoul Lufbery, became an ace), and ended the war with 14 aerial victories, all as a two-seater observer or pilot. He was the first American to earn the Military Cross, which was awarded by George V. Libby wrote a memoir titled Horses Don't Fly.

I know there were quite a few more Americans who served with the British air service(s), and I'm sure there were even more who served in the regular British Army and Royal Navy, but these are just the names I recall off the top of my head.

EDIT: Just found this link. Note that some pilots who joined the U.S. air service after the U.S. entered the war trained with RAF squadrons, but were ultimately assigned to the 17th or 148th Aero Squadrons which, although under the direction of the RAF, were technically American squadrons. However, some of those pilots served briefly in front-line RAF units before their assignment to the American squadrons.