The Australia, New Zealand, United States (ANZUS) security treaty was concluded at San Francisco on September 1, 1951, and entered into force on April 29, 1952. The treaty bound the signatories to recognize that an armed attack in the Pacific area on any of them would endanger the peace and safety of the others. It committed them to consult in the event of a threat and, in the event of attack, to meet the common danger in accordance with their respective constitutional processes. The three nations also pledged to maintain and develop individual and collective capabilities to resist attack.

In 1985, the nature of the ANZUS alliance changed after the Government of New Zealand refused access to its ports by nuclear-weapons-capable and nuclear-powered ships of the U.S. Navy. The United States suspended defense obligations to New Zealand, and annual bilateral meetings between the U.S. Secretary of State and the Australian Foreign Minister replaced annual meetings of the ANZUS Council of Foreign Ministers. The first bilateral meeting was held in Canberra in 1985. At the second, in San Francisco in 1986, the United States and Australia announced that the United States was suspending its treaty security obligations to New Zealand pending the restoration of port access. Subsequent bilateral Australia-U.S. Ministerial (AUSMIN) meetings have alternated between Australia and the United States. The 15th AUSMIN meeting took place in Washington on October 28, 2002.

The U.S.-Australia alliance under the ANZUS Treaty remains in full force. Defense ministers of one or both nations often have joined the annual ministerial meetings, which are supplemented by consultations between the U.S. Combatant Commander, Pacific and the Australian Chief of Defense Force. There also are regular civilian and military consultations between the two governments at lower levels.

ANZUS has no integrated defense structure or dedicated forces. However, in fulfillment of ANZUS obligations, Australia and the United States conduct a variety of joint activities. These include military exercises ranging from naval and landing exercises at the task-group level to battalion-level special forces training, assigning officers to each other's armed services, and standardizing, where possible, equipment and operational doctrine. The two countries also operate several joint defense facilities in Australia.

Following the terrorist attacks on the United States on September 11, 2001, Australian Prime Minister Howard invoked the ANZUS Treaty for the first time on September 14, 2001. Australia was one of the earliest participants in Operation Enduring Freedom. Australian Defense Forces participated in coalition military action against Iraq in Operation Iraqi Freedom. Australian military and civilian specialists are participating in the reconstruction of Iraq.

After a review in 2000 of Australia's future defense needs, the government made a commitment to increase defense spending by 3% a year for the next 10 years. The Australian Defense Force numbers about 49,500 active duty personnel. The Royal Australian Navy's front-line fleet currently includes 10 frigates, including six of the Perry class and four new Australian-built ANZAC class, with four more under construction. The last of six submarines of the new, indigenous Collins class was commissioned in March 2003. The F/A-18 fighter, built in Australia under license from the U.S. manufacturer, is the principal combat aircraft of the Royal Australian Air Force, backed by U.S.-built F-111 strike aircraft. In June 2002, Australia announced it intended to become a research and development partner in the U.S.-led Joint Strike Fighter program.