Enlargement of NATO
As of 2018, NATO officially recognizes four aspiring members: Bosnia and Herzegovina, Georgia, Macedonia and Ukraine. The Federation of Bosnia and Herzegovina within Bosnia and Herzegovina has expressed willingness to join NATO, however, it faces consistent political pressure from Republika Srpska, the other political entity in the country, alongside its partners in Moscow. Macedonia has been prevented from joining the alliance by Greece, one effect of the Macedonian naming dispute. Future expansion is currently a topic of debate in several countries outside the alliance, and countries like Sweden, Finland, and Serbia have open political debate on the topic of membership, while in countries like Ukraine, support and opposition to membership is tied to ethnic and nationalist ideologies. The incorporation of countries formerly part of the Eastern Bloc and the Soviet Union has been a cause of increased tension between NATO countries and Russia.
Montenegro
Montenegro was a member of NATO's Partnership for Peace program and then became an official candidate for full membership in the alliance. Montenegro applied for a Membership Action Plan on 5 November 2008, which was granted in December 2009. Montenegro is also a member of Adriatic Charter. Montenegro was invited to join NATO on 2 December 2015 and on 19 May 2016, NATO and Montenegro conducted a signing ceremony at NATO headquarters in Brussels for Montenegro's membership invitation. Montenegro became NATO's 29th member on 5 June 2017, despite Russia's objections. The government plans to have the army participate in peacekeeping missions through the UN and NATO such as the International Security Assistance Force.