This article looks at relations between Britain, the United States and Canada in the years leading up to the Second World War in order to ascertain the extent to which a North Atlantic Triangle can be said to have existed at the outbreak of war in September 1939. Drawing upon the author’s contention that an Anglo-American ‘tacit alliance’ was formed against Germany, Italy and Japan during President Franklin Roosevelt’s second term, it argues that the Canadian Prime Minister, William Lyon Mackenzie King, played an important part in this development by virtue of Canada’s position as the northern neighbour of the United States and the senior Dominion of the British Empire and that this ‘tacit alliance’ went hand in hand with a ‘North Atlantic Triangle’ between these three governments. The article first analyses the evolution of Mackenzie King’s relationships with Franklin Roosevelt and Neville Chamberlain in the 1930s. It then examines three key elements in the triangular relationship between Canada, the United States and Britain in 1938–9: the conclusion of an Anglo-American trade agreement in 1938; British appeasement policy and Roosevelt’s role during the Munich crisis of 1938; and the British Royal Visit to the United States in June 1939.