The Pioneer Corps was the only British unit that enemy aliens could serve in early on in the war. Many thousands of Germans and Austrians joined the Pioneer Corps to assist the Allied war efforts and liberation of their home countries. These were mainly Jews and political opponents of the Nazi Regime who had fled to Britain while it was still possible, and included the cinematographer Ken Adam, writer George Clare and publisher Robert Maxwell. These men - often dubbed "The King's Most Loyal Enemy Aliens" - later moved on to serve in fighting units. Some were recruited by Special Operations Executive (S.O.E.) as secret agents.