>https://history.state.gov/milestones/1945-1952/anzus
The Australia, New Zealand and United States Security Treaty (ANZUS Treaty), 1951
At the onset of the Second World War, both Australia and New Zealand were members of the British Empire, so when Britain entered the conflict they did as well. During the course of the war, these two geographically isolated nations faced the threat of direct attack for the first time in their histories as modern states.
As the other Allied powers turned their attention to the reconstruction of postwar Europe and Japan in the post World War II era, the governments of Australia and New Zealand remained concerned about the possibility of future Japanese expansionism and grew apprehensive about the rise of communism, particularly in East Asia. Even before the war ended, Australia and New Zealand signed an agreement stating that they had common goals and would work together in the international arena; at the time, the agreement was for both nations the first treaty negotiated independently,
and it reflected the concern that the major powers of the United States and the United Kingdom may not take Australian and New Zealander issues into account in their postwar planning.
In April of 1951, U.S. President Harry Truman announced that negotiations on a tripartite security treaty between the United States, Australia and New Zealand would occur concurrently with the negotiations for a final peace treaty with Japan. Both treaties were concluded in mid-1951, and the ANZUS Treaty was ratified by the United States and entered into force in 1952.
Although Britain also maintained an obligation to help defend Australian and New Zealander security as the leader of the Commonwealth, it was not invited to join the agreement.
In 1984, the ANZUS Treaty began to unravel when New Zealand declared its country a nuclear-free zone and refused to allow U.S. nuclear-powered submarines to visit its ports. Two years later, U.S. Secretary of State George P. Shultz and Australian Foreign Minister Bill Hayden concluded a series of bilateral talks by confirming that their countries would continue to honor their obligations to one another under the ANZUS Treaty, in spite of the fact that the trilateral aspects of the Treaty had been halted. On September 17, 1986, the United States suspended its treaty obligations toward New Zealand.