In 1949 the USSR detonated its first atomic bomb and Mao Zedong won power in China. A year later North Korea invaded South Korea and in 1951, during the Korean War, the Liberal government of Robert Menzies tried to ban the Communist Party of Australia, first by legislation that was declared invalid by the High Court, then by referendum to try to overcome the constitutional obstacles to that legislation. The 1951 referendum was opposed by the Communist Party as well as the Australian Labor Party, and was narrowly defeated. The issue of communist influence in the unions remained potent and led to the Australian Labor Party split of 1955 and the formation of the Democratic Labor Party comprising disaffected ALP members who were concerned over communist influence in Australian unions.