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.
Foreign policy of Vladimir Putin
The Baltic states of Estonia, Lithuania, and Latvia—all three of which are members of NATO—have warily watched Russian military movements and actions. All three countries, within the Russian Empire prior to 1918, had been annexed by the Soviet Union in 1940 as part of the Molotov–Ribbentrop Pact, and Russian leaders were particularly distressed by their accession to NATO and the EU in 2004. In 2014, the Baltic states reported several incursions into their air space by Russian military aircraft. Tensions rose as Russian intelligence forces crossed the Estonian border and captured Estonian intelligence officer Eston Kohver. In October 2014, Sweden engaged in a hunt for a foreign submarine that had entered its waters; suspicions that the submarine was Russian have caused further alarm in the Baltic states. However, the Swedish Armed Forces later admitted that the alleged sighting of a Russian sub was actually just a "workboat". The tensions in the Baltic and other areas have led neighboring Sweden and Finland, both of which have long been neutral states, to openly discuss joining NATO.