Foster got on well with the Americans. Isaiah Berlin, who was in Washington, DC in 1940, said that Foster was "very, very popular and never had less than two or three hundred friends." The United States awarded him the Legion of Merit during his service with the British Embassy. By 1944, he had established himself as a capable attorney with a broad and deep understanding of the international law. That year, he was appointed chief of the legal section in General Dwight Eisenhower's, Supreme Headquarters Allied Expeditionary Force (SHAEF). In order to facilitate his relationships with the military forces, he was given the rank of brigadier. Foster later quipped: "One must start somewhere." In addition to the American Legion of Merit, he was awarded the Legion of Honour and the Croix de guerre (with palms) from France.