The Cold War intensified after 1949 as China fell to communist rule under Mao Zedong, betraying American-Chinese relations. The Soviet Union further surprised the United States by acquiring atomic weapons earlier than expected. In 1950, North Korea, backed by the Soviet Union, invaded South Korea, which was under American influence. This started the Korean War and threatened to overrun the entire Korean peninsula until the United States intervened, leading to a costly three-year war that involved China on North Korea's side. The Korean War became a proxy conflict in the Cold War between communist and capitalist influences in North and South Korea respectively.