At the next Imperial Conference in 1926, King proposed that Governors General should no longer be the channels of diplomatic communication between London and the Dominions. He won this argument too, which led to the eventual posting of High Commissioners in the various Commonwealth capitals. A statement was issued that the Dominions were autonomous, equal communities within the British Empire. The same year, Canada sent her first ambassador to Washington.

Churchill's Wilderness Years began in the spring of 1929, when his Conservatives lost the election; in Canada in 1930, King's Liberals lost to the Conservatives under R.B. Bennett. This had the beneficial effect of leaving their opponents, Ramsay MacDonald and Bennett respectively, to face the economic depression.

In 1935, the outs became the ins: King was again Canada's premier, and Stanley Baldwin was Britain's. But Churchill was now waging his lonely fight against the British government's appeasement of Nazi Germany, and King by his very nature favoured conciliation. He was therefore supportive of Baldwin and his successor, Neville Chamberlain.

By 1937, Britain was slowly rearming, yet Mackenzie King at that year's Imperial Conference resisted efforts to pledge Canada's aid in case of war. With the blessing of Chamberlain, King visited Hitler, and his subsequent comments suggested that he like Chamberlain and others had been hoodwinked: "I do believe that it will be found that Hitler is for peace, unless unduly provoked." But he told Hitler that if Britain and Germany went to war, Canada would be at Britain's side.

Britain declared war on Germany on 3 September 1939. Four days later, the Canadian Parliament debated their own declaration. King put it to them "When it comes to a fight between good and bad, when the evil forces of the world are let loose upon mankind, are those of us who believe in Christianity and all it means going to allow evil forces to triumph, without if necessary opposing them by our very lives?" On 10 September, George VI, as King of Canada, approved the Canadian declaration of war on Germany.

On 1 October Churchill, now First Lord of the Admiralty, made his famous broadcast on the actions of Russia, " a riddle wrapped in a mystery inside an enigma." Then he added: "We have the freely-given ardent support of the twenty millions of British citizens in the self-governing Dominions of Canada, Australia, New Zealand, and South Africa." King cabled: "Your broadcast magnificent-as perfect in its appeal to the old world as to the new."

In 1930 Churchill had called Canada "a magnet exercising a double attraction drawing both Great Britain and the United States towards herself and thus drawing them closer to each other..... In fact, no state, no country, no band of men can more truly be described as the linch-pin of peace and world progress."

Ten years later Churchill's description seemed to prove out when King became a communicator between FDR and Churchill.