During the world wars and interwar period, 1914–1947, Canada experienced economic gain, more freedom for women, and new technological advancements. There were severe political tensions over issues of war and ethnicity, and heavy military casualties. The Great Depression hit Canada hard, especially in export-oriented mining and farming communities, and in urban factory districts.

World War I

The July Crisis was a series of interrelated diplomatic and military escalations among the major powers of Europe in the summer of 1914, which led to the unexpected outbreak of World War I (1914–1918). At the time, Canadians were more concerned with events within their own country than European affairs. The summer of 1914 brought a second year of drought turning wheat fields into parched deserts while the two new transcontinental railways the Grand Trunk Pacific and the Canadian Northern fell further into debt, sending the thousands of men who had helped build them into unemployment. Canada was facing its worst depression since the 1890s. Canadians hoped the Great Powers of Europe could keep the peace as they had done many times before in earlier disputes of the century. Besides, so far Britain had no reason to join in the squabble forming on the main continent, leaving no obligation for Canada to join if war did break out between Russia and France on one side and Germany and Austria-Hungary on the other. News of war did not make a stir in Canada until Germany invaded neutral Belgium, and the British delivered an ultimatum to withdraw from Belgium by August 4 or Britain would be at a state of war with Germany.

On July 29, 1914, Britain warned its colonies to take precautions in case of war. Most recent wars had begun with surprise attacks such as the Russo-Japanese War. Soldiers and Canada's few sailors manned Halifax fortifications and brought guns to command the St. Lawrence river. In Victoria, British Columbia Premier Richard McBride signed a cheque for $1,150,000 and bought two submarines from a Seattle shipyard, so at least British Columbia's coast was not completely defenceless. On August 2 armed militia mounted guard on bridges, canals, tunnels and railway stations in preparation. In Ottawa, the Minister of Militia, Sam Hughes, had dreamed for years of leading Canadians to war and had for a long time preached and prepared for war with Germany and now had only to wait on London to make the first move, much to his irritation, but was persuaded by the quartermaster general Major-General Donald Alexander Macdonald to be patient. On August 4, 8:55 P.M., Canada got the news and Hughes was ecstatic: Britain was at war with Germany.

Canada was then automatically at war, as she did not yet have control over her foreign policy — not that there were many dissenters. The war was initially popular even among French Canadians, including Henri Bourassa, who historically looked afoul at the British Empire. Liberal leader Wilfrid Laurier created a 'party truce' for as long as Canada was in danger and had those dissenters in the liberal caucus hold their tongues. When asked what Canada 'must do' by the press, Laurier responded "When the call comes, our answer goes at once, and it goes in the classical language of the British answer to the call of duty, 'Ready, aye, ready!'" Prime Minister Robert Borden called a meeting of Parliament on August 18, and without division or significant debate, MPs approved an overseas contingent of 25,000 men with Canada bearing the full cost: a war appropriation of $50 million and a Canadian Patriotic Fund to support the families of men who would fight in Europe. The Cabinet spent many hours trying to devise adequate emergency legislation, resulting in the War Measures Act, decreeing the Cabinet would have the authority to do whatever it deemed necessary for the security, defence, peace, order and welfare of Canada.

In no way was Canada prepared for this scale of war. Its economy could not support it for more than a few months before being hit hard by its cost, as was with other participants. No one expected it to last longer than a few months though, many claiming it would be over by Christmas. Mass recruiting for the war effort began on August 6 with hundreds of telegrams notifying Militia colonel to begin recruiting men between the ages of 18 and 45. Hordes of British immigrants and the unemployed answered the call. Ontario, hard hit by the depression, accounted for third of the recruits, while two thirds of the recruits were British born. Few recruits came from the Maritimes and just over 1,000 were French. The cities of Toronto, Winnipeg and Montreal sent enough men each for two battalions. By September 4 there were 32,000 men and 8,000 horses in camp, far more than had been expected. There was an immediate demand for equipment, uniforms and weapons. The Ross Rifle Company worked overtime as did the textile mills and clothing factories. With a force of 32,000 equipped and ready, it soon became apparent that Embarkation from the docks would be a nightmare. Extra ships had to be chartered to carry the additional men. Battalions were marched on to ships only to be marched back off when they didn't fit. Units ignored orders and schedules and crowded the docks not wishing to wait. When it was all done, the last of 30 ships had cleared the harbour, leaving 863 horses, 4,512 tons of baggage, vehicles and ammunition behind, for which another ship had to be called in to pick up.

The first Canadian casualties of the war occurred before these troops arrived in Europe. Christopher Cradock's squadron was sunk at the Battle of Coronel off the coast of Chile, claiming four midshipmen who became Canada's first war dead. By the time that the First Contingent reached England on October 14 it became apparent the war would not be over by Christmas. Germany's initial rapid successes in Belgium and France had come to halt and both sides were starting to dig into their positions.

Canadians fought at Ypres, the Somme, Passchendaele, and other important battles, originally under British command, but eventually under a unified Canadian command. From a Canadian point of view the most important battle of the war was the Battle of Vimy Ridge in 1917, during which Canadian troops captured a fortified German hill that had eluded both the British and French. Vimy, as well as the success of the Canadian flying ace Billy Bishop, helped give Canada a new sense of identity. With mounting costs at home, Minister of Finance Thomas White introduced the first income tax in Canada as a "temporary" measure. The lowest bracket was 4% and highest was 25%.

The 620,000 men in service were most remembered for combat in the trenches of the Western Front; there were 67,000 war dead and 173,000 wounded. This total does not include the 2,000 deaths and 9,000 injuries in December 1917 when a munitions ship exploded in Halifax.War Office, Statistics of the Military Effort of the British Empire During the Great War 1914–1920 (London, 1922) p. 237

The conscription crisis of 1917

After three years of a war that was supposed to have been over in three months, Canada was suffering from a shortage of volunteers. Prime Minister Robert Borden had originally promised not to introduce conscription, but now believed it was necessary to win the war. The Military Service Act was passed in July, but there was fierce opposition, mostly from French Canadians (led not only by Bourassa, but also by Wilfrid Laurier), as well as Quakers, Mennonites, and other pacifists. Borden's government almost collapsed, but he was able to form a Union government with the Liberal opposition (although Laurier did not join the new government). In the 1917 election, the Union government was re-elected, but with no support from Quebec. Over the next year, the war finally ended, with very few Canadian conscripts actually participating.

Halifax Explosion

Halifax, Nova Scotia, was the main staging point for convoys making trans-Atlantic crossings. On December 6, 1917, a Belgian relief ship collided with the , a French munitions ship in Halifax harbour. The crash set the Mont-Blanc on fire; its holds were full of benzol, picric acid, and TNT. Twenty minutes later it exploded with a force stronger than any man-made explosion before it, destroying most of Halifax and the surrounding towns. Out of a population of 50,000, 1600 people were killed and over 9,000 injured; hundreds were blinded by flying glass. The city was evacuated and dropped out of the war effort, focusing primarily on economic survival.

Post-war society During the war, the woman's suffrage movement gained support. The provinces began extending voting rights to women in 1916, and women were finally allowed to vote in federal elections in 1918. Canada was also faced with the return of thousands of soldiers, with few jobs waiting for them at home. They also brought back with them the Spanish flu, which killed over 50,000 people by 1919, almost the same number that had been killed in the war.

Labour conflicts

The move from a wartime to a peacetime economy, combined with the unwillingness of returned soldiers to accept pre-war working conditions, led to another crisis. In 1919, the One Big Union was formed by trade union syndicalists with the intent of improving conditions for all workers, not just in a single workplace, industry, or sector. The OBU had some influence on the Winnipeg General Strike of 1919, which business and political leaders saw as an outbreak of Bolshevism, especially since the Soviet Union had recently been formed. The army was sent in to break the strike and the entire Winnipeg police force was fired and replaced with a much larger and better paid force of armed special constables. Although the Winnipeg strike is the best known, it was part of a larger strike wave that swept the country between 1919 and 1925. Special constables, vigilante "citizens" organizations, and replacement workers were mobilized in strikebreaking throughout the country in this period.Trevor Stace, "Remembering and Forgetting Winnipeg: Making History on the Strike of 1919." Constellations (2014) 5#1 on the historiography of 1919; online .Donald Campbell Masters, The Winnipeg general strike (University of Toronto Press, 1950) is a scholarly history.

Politics Meanwhile, in western Canada, and to some extent in the Maritimes, populist reformers were pushing for increased provincial rights and a focus on agriculture, rather than the industrial focus of Central Canada. They formed the Progressive Party of Canada, which supported Mackenzie King when the Liberals had a minority government in 1925–26. King eventually lost support, however, because of the trade tariffs issue, as well as a liquor smuggling scandal. When his request that parliament be dissolved was rejected by the Governor General of Canada (see King-Byng Affair), he was forced to resign in 1926, but was re-appointed after his party won the election later that year, after which, at an Imperial Conference, King advocated the redefining of the role of the Governor General and the gain of increased independence for Canada in the Balfour Declaration of 1926.

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