<Tr> <Th> Expenses (2017) </Th> <Td> US $946 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 29 North American and European countries . The organization implements the North Atlantic Treaty that was signed on 4 April 1949 . NATO constitutes a system of collective defence whereby its independent member states agree to mutual defence in response to an attack by any external party . NATO's Headquarters are located in Haren, Brussels, Belgium, while the headquarters of Allied Command Operations is near Mons, Belgium . </P> <P> Since its founding, the admission of new member states has increased the alliance from the original 12 countries to 29 . The most recent member state to be added to NATO is Montenegro on 5 June 2017 . NATO currently recognizes Bosnia and Herzegovina, Georgia, Macedonia and Ukraine as aspiring members . An additional 21 countries participate in NATO's Partnership for Peace program, with 15 other countries involved in institutionalized dialogue programs . The combined military spending of all NATO members constitutes over 70% of the global total . Members have committed to reach or maintain defense spending of at least 2% of GDP by 2024 . </P>

Which alliance was formed as a reaction to the north atlantic treaty organization (nato)