This thesis identified the role played by the North Atlantic Treaty Organization (NATO) in the eastern part of Europe through its Open-Door policy. It measured the extent to which this policy contributed to the stability and security of the European continent in general and the candidate countries that wish to join NATO in particular. The thesis studied the security dilemma of the post USSR countries in Eastern Europe, particularly those in the middle of two antagonistic blocs (NATO vs. Russian Federation). For these countries, joining either bloc meant one thing, upsetting the other one. The thesis shed the light on the ongoing Russo-Ukrainian war to measure the hypothesis of the study, by scrutinizing the evolution of Russia-Ukraine relations, and Russia western relations, to clarify the security dilemma that appeared because of NATO's expansion in Eastern Europe. The thesis relied on two theoretical frameworks, the Structural Realist theory, and the theory of the Regional Security Complex (RSCT). Also, the thesis opted for descriptive analytical approach to highlight the impact of NATO's open-door policy and its security implications on the region in general, and on Ukraine in particular. The thesis concluded that the historical hostility between NATO and USSR (represented by Warsaw Pact) during the Cold War era reflected in today's antagonism between NATO and Russia and was exaggerated by NATO's expansion in Russia sphere of influence (post-USSR countries), such as Ukraine. The strategic location of Ukraine (separates NATO from Russia), and its historical relations with Russia have made it difficult to join a western security alliance, because it threatens the security of Russia. The thesis continued on the previous scholastic work, by introducing solution alternatives to end the tensions between NATO's expansion, and Russia's security concerns. As well as, suggesting potential political and security behaviors for Eastern Europe countries, to avoid being pulled in conflict.