With tensions over the Polish situation increasing during August, the British government kept Canada fully apprised of the situation through a series of telegrams between London and Ottawa. The Department of National Defence sent letters of instruction to all Militia District Offices regarding mobilization plans. Although Canada's material preparations for war may have been grossly deficient, the mobilization plan was well thought out and widely disseminated.
On 23 August 1939, the Canadian prime minister, William Lyon Mackenzie King, announced that provisions of the War Measures Act would be enacted if necessary. On 25 August, 99 militia units (a figure later increased to 106), were placed on active duty to guard designated military and civilian vital points. These guards were in place by the 27th. At the same time, the government cancelled leave for all officers and men of the permanent force. By 31 August, more than 10,000 militiamen had reported for service as part of these domestic defensive measures.
On 1 September, on confirmation that the German army had invaded Poland, the government mobilized the Mobile Force, the name given to the two-division expeditionary force provided for in the mobilization plan. Although previously designated units were ordered onto active service, individual unit members were not compelled to serve. Each soldier had to volunteer for active duty on being called in by his unit. Parliament approved this action on 10 September, the same day that Canada formally declared war. On 23 September, the government limited the expeditionary force to the 1st Canadian Infantry Division, and recruiting for the 2nd Division therefore was temporarily suspended. The division, however, was kept intact. By the end of September, 60,000 men had volunteered for service, and the 1st Canadian Division was in Britain by December. Canada went to war with many grave deficiencies in terms of modern armaments, but at least the army general staff had done their homework and produced a workable, well-thought out mobilization that worked superbly in September 1939.