In 1951, during the early stages of the Cold War, Menzies spoke of the possibility of a looming third world war. The Menzies Government entered Australia's first formal military alliance outside of the British Commonwealth with the signing of the ANZUS Treaty between Australia, New Zealand and the United States in San Francisco in 1951. External Affairs Minister Percy Spender had put forward the proposal to work along similar lines to the NATO Alliance. The Treaty declared that any attack on one of the three parties in the Pacific area would be viewed as a threat to each, and that the common danger would be met in accordance with each nation's constitutional processes. In 1954 the Menzies Government signed the South East Asia Collective Defence Treaty (SEATO) as a South East Asian counterpart to NATO. That same year, Soviet diplomat Vladimir Petrov and his wife defected from the Soviet embassy in Canberra, revealing evidence of Russian spying activities; Menzies called a Royal Commission to investigate.

Which treaty was Australia's first military alliance outside of the British Commonwealth?