Growing commercial and industrial prosperity attracted large scale immigration from the British Isles,
The steel industry continued to grow and consolidate, resulting in the Steel Company of Canada (Stelco) in 1910 and Dominion Steel Casting Company (Dofasco) in 1912, who located in the city's north end to be close to the water for both transportation and cooling.
Following the turn of the century, immigration continued, from black Americans, Italians, who sertled along Murray Street ("Corso Raculmuto"), and Austria-Hungary. During World War I, Hamilton sent four battalions of the Royal Hamilton Light Infantry who fought in France as apart of the Canadian Expeditionary Force.
The Hamilton Board of Education continued building new schools, many named for the war veterans: Memorial School, Allenby School and Earl Kitchener School. Hamilton's municipal government, civic boosters and ordinary residents lured McMaster University, funded by a bequest of Senator William McMaster in 1887, from Toronto with grants of land and money in 1930.
That same year Hamilton hosted the inaugural Empire Games, now the Commonwealth Games, allowing amateur athletes from around the British Empire and Commonwealth to compete at Hamilton Civic Stadium, the current site of Ivor Wynne Stadium.
The Great Depression of the 1930s hit Hamilton hard, affecting the steel industry specifically as well as all other consumer and industrial goods. Make-work government projects designed to prime the economy, but also began to improve the long-term attractiveness of Hamilton. Thomas B. McQuesten, a Hamilton lawyer and MLA, who served as minister of transportation and as chairman Niagara Parks Commission starting in 1934 spearheaded the construction of the Queen Elizabeth Way, Canada's first controlled access highway, to link Fort Erie with Toronto via Hamilton. He also supported the construction of the Rock Gardens at the Royal Botanical Gardens.
When the Second World War began, Hamilton quickly mobilized to send troops, including a number who took part in , and died in the the fateful raid on Dieppe, as a testing ground for tactics used on D-Day. Hamilton also played a key role in the British Commonwealth Air Training Plan, with a Royal Canadian Air Force training station at Mount Hope (now the location of its international airport). The War, with its shortage of men, also led to the introduction of women into the paid industrial workforce.
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