In May and June 1939, the King and Queen toured Canada and the United States. From Ottawa, the royal couple were accompanied throughout by Canadian Prime Minister William Lyon Mackenzie King, to present themselves in North America as King and Queen of Canada. George was the first reigning monarch of Canada to visit North America, although he had been to Canada previously as Prince Albert and as Duke of York. Both Governor General of Canada Lord Tweedsmuir and Mackenzie King hoped that the King's presence in Canada would demonstrate the principles of the Statute of Westminster 1931, which gave full sovereignty to the British Dominions. On 19 May, George VI personally accepted and approved the Letter of Credence of the new U.S. Ambassador to Canada, Daniel Calhoun Roper; gave Royal Assent to nine parliamentary bills; and ratified two international treaties with the Great Seal of Canada. The official royal tour historian, Gustave Lanctot, wrote "the Statute of Westminster had assumed full reality" and George gave a speech emphasising "the free and equal association of the nations of the Commonwealth".