<P> The beginning of World War II in Europe and the threat of the U.S. becoming involved generated new jobs in defense - related industries . After the U.S. became involved in the war in 1941, the city mobilized . Philadelphia consistently met war bond quotas and when the war ended in 1945, 183,850 residents were in the U.S. armed forces . With so many men serving in the military, there had been a labor shortage; businesses and industries hired women and workers from outside the city . In 1944 the Philadelphia Transportation Company promoted African Americans to positions as motormen and conductors (from which they had previously been excluded) on public transportation vehicles . Resentful, other PTC workers protested and began a strike that nearly immobilized the city . President Roosevelt sent troops to replace the striking workers . After a federal ultimatum, the workers returned after six days . </P> <P> After World War II ended, Philadelphia had a serious housing shortage . Around half of the city's housing had been built in the 19th century, and many units lacked proper sanitary facilities, were overcrowded, and in poor condition . Competition for housing, as African Americans (many had come to the city in the Great Migration from the South) and Puerto Ricans moved into new neighborhoods, resulted in racial tension . The wealthier middle - class residents, often white, continued to move out to the suburbs in what became called white flight . </P> <P> The population peaked at more than two million residents in 1950; afterward the city's population declined while that of the neighboring suburban counties grew . Some residents moved out of the region altogether due to restructuring of industry and loss of tens of thousands of jobs in the city . Philadelphia lost five percent of its population in the 1950s, three percent in the 1960s and more than thirteen percent in the 1970s . Manufacturing and other major Philadelphia businesses, which had supported middle - class lives for the working class, were moving out of the area or shutting down in industrial restructuring, including major declines in railroads . </P> <P> The city encouraged development projects in University City in West Philadelphia and the area around Temple University in North Philadelphia, it removed the "Chinese Wall" elevated railway, and developed Market Street East around the transportation hub . Some gentrification occurred, with restoration of properties in historic neighborhoods such as Society Hill, Rittenhouse Square, Queen Village, and the Fairmount area . A non profit group Action Philadelphia was formed to improve and promote Philadelphia's image. The airport expanded, the Schuylkill Expressway and the Delaware Expressway (Interstate 95) were built, SEPTA was formed, and residential and industrial development took place in Northeast Philadelphia . </P>

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