THE first half of the twentieth century saw the widespread emergence of large cities in the United States. In 1900, there were only six cities with more than half a million inhabitants; only 50 years later, there were 17 such cities. Much of this urban growth was stimulated by two world wars and the government-supported expansion of war-related industries, most located in big Northeastern and Midwestern cities. The largest cities also benefited from the fact that for more than a decade after the Second World War the United States was the only country in the world with its manufacturing facilities intact.