At the high tide of Britain’s twentieth century power, Sir Archibald Clark Kerr reached the pinnacle of his remarkable and controversial career. He served in vital diplomatic posts and was a major figure in determining and executing British foreign policy. An Australian-born Scot, he joined the Foreign service in 1906. His promising early career suffered a serious setback in 1924, however, when as deputy to Lord Allenby in Cairo he challenged the Foreign Office over its Egyptian policy. After exile in Central America, he returned to favour with the top posts in Sweden and Iraq. An outstanding Ambassador to China during the Japanese occupation of the late 1930s, he was elevated to Moscow as Ambassador in 1942. There he established a notable relationship with Stalin and was a senior British delegate at each of the ‘Big Three’ conferences. His vital work with Churchill, Roosevelt and Stalin is regarded as having been crucial in holding the wartime alliance together. After the war he was rewarded with the post of Ambassador to the USA where, as the Cold War grew, he participated in the births of both the Marshall Plan and NATO. Long judged suspect by the Establishment for his outspoken left-wing and anti-imperialist views, in recent times even his loyalty has been questioned. An acquaintance of Guy Burgess, and Donald Maclean’s superior in the Washington Embassy, he took their defection to Moscow badly, the shock hastening his early death in 1951. These and other big issues are fully explored in this text which is based on unique access to the vast Inverchapel archive.