The new Conservative government, headed by Robert Laird Borden, had the responsibility of rallying the nation to Britain's side in World War I. Had Canadians remained as divided as they were at the end of Laurier's term, this might have been a difficult thing to do. But Germany's invasion of neutral Belgium in 1914 forged a unity of Canadian sentiment and a demand for participation in the conflict.

The first Canadian contingent, numbering 33,000, reached England soon after the outbreak of war in 1914, and it was in the thick of the fighting on the continent a few months later in the second battle of Ypres. By 1916 the Canadians had formed four divisions, with a fifth to provide reinforcements. The four divisions of the Canada Corps earned an outstanding reputation as a fighting force. More significant, however, was the fact that Canada was playing a respectable role on the world stage, a role that would soon help undo its colonial status.

Before the war ended in 1918, more than 619,000 officers and men had enlisted, including some 22,000 who had served in the British Royal Air Force. More than 60,000 Canadians were killed in action or died of wounds, a terribly heavy toll in relation to the country's population. Over 66 million shells were produced in Canadian factories. The gross national debt soared from 544 million dollars in 1914 to almost 2 1/2 billion dollars in 1919, most of the money being raised in Canada itself through public war loans.

The Canadian forces at the outset were made up wholly of volunteers. Casualties and the rapidly accelerating pace of the war made the bitter question of conscription a major issue by 1917. Borden met it by forming a coalition government of Conservatives and Liberals, though Laurier refused to join the coalition. In the election of that year, Quebec was almost unanimous in its opposition to the conscription policy that was supported elsewhere across the country. The political solidarity of the province during the next 25 years was largely derived from its memory of that episode.

On the battlefronts in France and Belgium, Canadians of both nationality backgrounds made magnificent contributions to the final victory. They faced with heroism the first poison-gas attack in the history of warfare during the second battle of Ypres in 1915. Other engagements in which Canadian forces earned the admiration of all the Allies included the battles of Mount Sorrel (1916), the Somme (1916), and Vimy Ridge (1917). The victory of Passchendaele Ridge in the autumn of 1917 alone cost 16,000 Canadian casualties. In 1918 during the closing months of the war, Canadians again saw heavy action at Amiens, Cambrai, and Mons.