As Thailand allied with Japan in World War II, Bangkok was subjected to Allied bombing, but rapidly grew in the post-war period as a result of US aid and government-sponsored investment. Bangkok's role as a US military R&R destination boosted its tourism industry as well as firmly establishing it as a sex tourism destination. Disproportionate urban development led to increasing income inequalities and migration from rural areas into Bangkok; its population surged from 1.8 million to 3 million in the 1960s.