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.[8] American mercenary Colonel Charles Sweeny living in London began recruiting American citizens to fight as a U.S. volunteer detachment in the French Air force, however France fell before this was implemented.[8] During the Battle of Britain, 11 American pilots flew in the RAF, one of whom was killed.