Anzus Treaty (ăn`zəs), defense agreement signed in 1951 by Australia, New Zealand, and the United States. The name Anzus is derived from the initials of the three signatory nations. As a result of the reestablishment of peace between Japan and the United States in 1951, Australia and New Zealand asked for a treaty making it clear that an attack on any of the three signatory countries would be considered an attack upon all. The pact became effective in 1952. New Zealand's 1985 refusal to allow U.S. nuclear-powered or nuclear-armed ships to enter its ports caused the United States to abrogate its ANZUS responsibilities toward New Zealand in 1986; New Zealand has not formally withdrawn from the alliance.