The ANZUS Treaty between the United States, Australia, and New Zealand was signed in 1951, locking the three countries into a mutual defence pact. This increased social and political ties between Australia and the United States and led to Australia and New Zealand committing troops to the Vietnam War in the 1960s and 1970s – these connections and increased worldwide travel encouraged greater numbers of United States citizens to migrate permanently and in 1971 there were 39,035 United States-born residents in Australia