or supported another union in a specific strike. Their wagons could easily be positioned to disrupt streetcars and block traffic. In addition, their families and neighborhood supporters often surrounded and attacked the wagons of nonunion teamsters who were strikebreaking. When the teamsters used their clout to engage in sympathy strikes, employers decided to coordinate their antiunion efforts, claiming that the teamsters held too much power over commerce in their control of the streets. The teamsters' strike in 1905 represented a clash both over labor issues and the public nature of the streets. To the employers, the streets were arteries for commerce, while to the teamsters, they remained public spaces integral to their neighborhoods. World War II On December 2, 1942, the world's first controlled nuclear reaction was conducted at the University of Chicago as part of the top secret Manhattan Project. During World War II, the steel mills in the city of Chicago alone accounted for 20% of all steel production in the United States and 10% of global production. The city produced more steel than the United Kingdom during the war, and surpassed Nazi Germany's output in 1943 (after barely missing in 1942). The city's diversified industrial base made it second only to Detroit in the value—$24 billion—of war goods produced. Over 1,400 companies produced everything from field rations to parachutes to torpedoes, while new aircraft plants employed 100,000 in the construction of engines, aluminum sheeting, bombsights, and other components. The Great Migration, which had been on pause due to the Depression, resumed at an even faster pace as the 1910 - 1930 period, as hundreds of thousands of black Americans arrived in the city to work in the steel mills, railroads, and shipping yards. Postwar Returning World War II veterans and immigrants from Europe (in particular displaced persons from Eastern Europe) created a postwar economic boom and led to the development of huge housing tracts on Chicago's Northwest and Southwest sides. The city was extensively photographed during the postwar years by street photographers such as Richard Nickel and Vivian Maier. In the 1950s, the postwar desire for new and improved housing, aided by new highways and commuter train lines, caused many middle and higher income Americans to begin to move from the inner-city of Chicago to the suburbs. Changes in industry after 1950, with restructuring of the stockyards and steel industries, led to massive job losses in the