The treaty was one of the series that the United States formed in the 1949–1955 era as part of its collective response to the threat of communism during the Cold War. New Zealand was suspended from ANZUS in 1986 as it initiated a nuclear-free zone in its territorial waters; in late 2012 New Zealand lifted a ban on visits by United States warships leading to a thawing in tensions. New Zealand maintains a nuclear-free zone as part of its foreign policy and is partially suspended from ANZUS, as the United States maintains an ambiguous policy whether or not the warships carry nuclear weapons and operates numerous nuclear-powered aircraft carriers and submarines; however New Zealand resumed key areas of the ANZUS treaty in 2007.