The Collective Security Treaty Organization (CSTO) is an Eurasian military alliance created in 2002. On one hand the Organization was developed with the aim to counter threats present in the region; on the other, its members continue to struggle with a plethora of state, transnational and domestic menaces. Thus, in four instances its members requested military assistance: Kyrgyzstan in 2010, Tajikistan in 2021, Kazakhstan and Armenia in 2022. However, only in the case of Kazakhstan CSTO granted its request and intervened. As a result, one might wonder why the organization is unable to provide effective security to its members besides their own requests of help. Hence, this thesis attempts to find explanations for ineffectiveness in alliances built on collective defense mechanisms. With this aim, this qualitative work makes a comparative analysis of the cases, using the Most Similar System Designs (MSSD) in order to trace a pattern which explains the observed divergent outcomes. Consequently, interviews with seven experts were made in order to gather data for the analysis. The MSSD analysis spotted four relevant factors, with two of them being determinant for the outcomes. Nonetheless, an alternative analysis, unable to be done with the same design, had other significant findings. By utilizing the rationalist approach of International Relations theory in order to interpret the results, the research showed two complementary rather than differing frameworks: the first which focused on the common consideration of threat by the alliance as well as the Russian interest in the matter, and the second concentrated on the other member states’ interests and in the context they operate. This thesis concludes that the best explanation consists of binding those two approaches in order to provide an answer for the research question.