NATO's effect on European security structure in wake of Ukrainian crisis: A research on Baltic security

International order after the Second World War; It was established by the division of the world into two rival alliances, militarily and politically, between the USSR and the USA. During this period, which is called the Cold War, the USSR kept the Eastern Bloc under its political and military control through the Warsaw Pact, while the USA kept the Western Bloc under its political and military control through NATO. With the end of the cold war, a security environment was established between the former rival states in Europe. This environment ended with Russia's illegal annexation of Crimea and its covert military intervention in the east of Ukraine. Russia has emerged as a clear threat to stability in Europe. The EU and NATO have reorganized in order to increase the effectiveness of cooperation between them in order to ensure stability in Europe. In the research, After the Ukraine crisis, the impact of NATO on the European Security Structure and the future direction of that Structure were investigated in the light of the Balance of Threat theory. In this context, the foreign policies of the Baltic states towards the EU, NATO and Russia and the reasons for theirs EU and NATO membership preferences are discussed. After the Ukraine crisis; The change in the security perceptions of NATO and EU states, the joint and individual measures taken to ensure European security, the scope of defense cooperation between the EU and NATO and possible alternatives are presented. Key Words: Balance of Threat Theory, Baltic Countries, Cold War, ESS, NATO, Common Security and Defence Policy, Ukraine Crisis.