In addition to the 16 million Americans who left home for military service during World War II, another 15 million changed their residence. Most people moved for new jobs in war plants. As a result of the population movement, 'boomtowns' appeared throughout the nation, and with them came slums. In Washington, DC, the population grew from 900,000 in 1940 to almost 1.3 million by the summer of 1943.