Notes: Montenegro is set to become the 29th member of the North Atlantic Treaty Organization (NATO) after US Senate ratify its entry into NATO. Though, ratification from Spain and the Netherlands is still pending ahead of a NATO summit in May 2017. The move was strongly opposed by Russia because it considers Montenegro and other western Balkan states part of its sphere of interest. Thus, Russia opposes the Western military alliance’s expansion in the Balkans. It must be noted that Montenegro was the former ally of Russia. Under NATO’s principle of collective defense, an attack against one ally is considered an attack against them all.