From Americans at War in Foreign Forces: [In 1940], and after much back and forth within the British government, King George VI approved a decision that an oath of allegiance to Britain would not be required of these Americans. And, lest there be any uncertainties on the matter, it was promised that any American who lost the citizenship of his country would be offered British citizenship in its place.
On the other side of the Atlantic, the King’s action was met with an unpublicized decision on the part of General Lewis B. Hershey, director of the U.S. Selective Service. Decades later, Hershey would become a focus of protest against America’s participation in the Viet Nam war, but he was a strong manager of the draft during the years of World War II. In July 1941, All of the state’s 330 local draft boards, it said, had received a directive from Hershey stating that it was national policy to “lend all aid to the Canadian and British governments.” Therefore, those who enlisted in those forces – whether military, medical or technical - were to be given the deferment classification of 11B for those who were engaged in crucial national defense work. Further, though the 11B deferment had to be renewed each six months, these particular deferments would renew in perpetuity as long as their holders remained in service, or until they were recalled for service in the defense of their own country. The men who had already so enlisted had previously been noted as delinquents serving in the Commonwealth forces. The condition that they not be required to take an oath of allegiance to a foreign country, however, erased that notation and gave them full standing as Americans fulfilling an American policy. It was agreed on all sides that Canada and England would return the men to America if the need for them arose.