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.
Permanent Structured Cooperation
About four-fifths of PESCO members are also member states of NATO, and two EU states (Denmark and the UK) that are members of NATO are not members of PESCO. While PESCO was formed in part due to doubts over US commitment to NATO, officials stress that PESCO will be complementary to NATO security rather than in competition with it. NATO is still viewed as the main guarantor of Europe's defence, while PESCO focuses on crisis deployments. NATO Secretary General Jens Stoltenberg also highlighted how Military Mobility is a key example of NATO and EU co-operation.