<Tr> <Th> Expenses (2016) </Th> <Td> $892 billion </Td> </Tr> <Tr> <Th> Website </Th> <Td> nato.int </Td> </Tr> <P> The North Atlantic Treaty Organization (NATO / ˈneɪtoʊ /; French: Organisation du Traité de l'Atlantique Nord; OTAN), also called the North Atlantic Alliance, is an intergovernmental military alliance between several North American and European states based on the North Atlantic Treaty that was signed on 4 April 1949 . </P> <P> NATO constitutes a system of collective defence whereby its member states agree to mutual defence in response to an attack by any external party . Three NATO members (the United States, France and the United Kingdom) are permanent members of the United Nations Security Council with the power to veto and are officially nuclear - weapon states . NATO Headquarters are located in Haren, Brussels, Belgium, while the headquarters of Allied Command Operations is near Mons, Belgium . </P>

What do the letters in nato stand for