In February 2022, Russia launched a full-scale invasion of Ukraine to topple the Western-aligned government of Volodymyr Zelenskyy. The invasion escalated an eight-year conflict that began with Russia’s annexation of Crimea in 2014, breaking 50 years of peace in Europe and upending the security order. The conflict forced Eastern European states to respond, given their proximity to Russia and Ukraine, but these responses were not uniform. Some states firmly back Kyiv while others maintain ties with Moscow or balance between the two. This thesis seeks to understand why Eastern European states have adapted different security strategies in response to the War in Ukraine, given that they all face security threats from Russian expansionism. Four international relations theories – realism, institutionalism, constructivism, and domestic politics – offer different explanations of state behavior, and the thesis tests which one is most suited for this context. Case studies of Hungary, Poland, and Estonia find that realism best explains state responses. Consistent with the realist focus on survival and balance of power, states closer to Russia and with higher perceived threats to their sovereignty are more likely to engage in alliances and military actions to deter aggression. These findings suggest that security concerns shape secondary states’ foreign policy more than institutional or ideological commitments. With pressure to end the war and initiate peace talks in March 2025, Eastern European states play a key role in shaping regional stability. Per the findings, while Hungary may seek to normalize diplomacy with the Kremlin, Poland and Estonia will likely continue prioritizing collective security to deter Vladimir Putin for good. These predictions underscore that strategic threat assessment remains the primary driver of Eastern European foreign policy.