This thesis tests Charles A. Kupchan’s hypotheses about discord and cooperation to explain why Ukraine has not received a membership invitation from NATO, despite the promise made in 2008. The hypotheses are examined on the cases of the annexation of Crimea in 2014 and the Russian mobilization in 2021, focusing on the NATO-members United States, United Kingdom, Germany and France. The result shows that Kupchan’s hypotheses partly explain the discord and that the domestic politics hypothesis, through German energy dependence, French economic incentives and pursuit of strategic autonomy, combined with the alliance security dilemma, through restrained U.S. leadership carried the greatest explanatory weight. External threat hypothesis played some role, while collective action finds no empirical support. The study thus suggests that national, state-specific factors, rather than Russian aggression alone, help explain how NATO’s policy toward Ukrainian membership has been shaped.