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.
Russia–NATO relations
On 25 March 2014, Stoltenberg gave a speech to a Labour Party convention where he harshly criticized Russia over its alleged invasion of Crimea, stating that Russia threatened security and stability in Europe and violated international law, and calling Russia's actions unacceptable. After his election as NATO Secretary-General, Stoltenberg emphasized that Russia's invasion of Ukraine was a "brutal reminder of the necessity of NATO," stating that Russia's actions in Ukraine represented "the first time since the Second World War that a country has annexed a territory belonging to another country."
Stoltenberg has highlighted the necessity of NATO having a sufficiently strong military capacity, including nuclear weapons, to deter Russia from violating international law and threaten the security of NATO's member states. He has highlighted the importance of Article 5 in the North Atlantic Treaty and NATO's responsibility to defend the security of its eastern members in particular. He has further stated that Russia needs to be sanctioned over its actions in Ukraine, and has said that a possible NATO membership of Ukraine will be "a very important question" in the near future. Stoltenberg has expressed concern over Russia acquiring new cruise missiles.