During the Korean War, Eisenhower threatened to use nuclear weapons if the stalemate was not broken, helping end the war. China emerged as a new superpower. In response to tensions over islands, Eisenhower used brinkmanship and the threat of massive retaliation to reach a compromise between China and Taiwan. The US backed South Vietnam after France withdrew following its defeat at Dien Bien Phu, fearing communism would spread under the domino theory, but this commitment would grow larger over time. In Iran, the US returned the Shah to power through covert CIA action after the prime minister nationalized oil supplies, fearing Iran could become communist.