Prior to America’s entry into World War II in December 1941, individual Americans volunteered to fight against the Axis powers in other nations armed forces. Although under American law, it was illegal for United States citizens to join the armed forces of foreign nations and in doing so, they lost their citizenship, Many American volunteers changed their nationality to Canadian. However Congress passed a blanket pardon in 1944. American mercenary Colonel Charles Sweeny (see Sweeny) living in London began recruiting American citizens to fight as a US volunteer detachment in the French Air force, however France fell before this was implemented. During the Battle of Britain, 11 American pilots flew in the RAF, one of whom was killed, Charles Sweeney’s nephew, also called Charles formed a Home Guard unit from American volunteers living in London.
One notable example were the Eagle Squadrons , these were RAF Squadrons made up of American volunteers and British personnel. About 100 Eagle pilots had been killed, were missing, or were prisoners. The first squadron was formed; NO.71 Squadron on 19 September 1940 by followed by No. 121 Squadron on 14 May 1941 and No. 133 Squadron on 1 August 1941. 6,700 Americans applied to join but only 244 Americans got to serve with the three Eagle Squadrons,16 Britons also served as squadron and flight commanders. The first Eagle Squadron became operational in February 1941 and the Squadrons scored their first kill in July 1941. On 29 September 1942, the three squadrons were officially turned over by the RAF to the Eight Air Force of the U.S Army Air Forces and became the 4th Fighter Group, in their time with the RAF the squadrons claim to have shot, 73½ German planes between them, 77 Americans and 5 Britons were killed.
Another notable example is were Flying Tigers, created by Claire L Chennault (see Chennault), a retired U.S. Army Air Corps officer working in the China since August 1937, first as military aviation advisor to Generalissimo Chiang kai Chek (see Chiang kai Chek) in the early months of the Sino Japanese War, a military conflict fought primarily between the Republic of China and the Empire of Japan from 1937 to 1941. During their time in the Chinese Air Force, they succeeded in destroying 296 enemy aircraft, while losing only fourteen pilots in combat.