Parallel expansions of the European Union and the North Atlantic Treaty Organization (NATO) in recent decades have blurred the lines between membership in the two organizations and the conditionalities attached to each. While both organizations have seemed successful in attracting Eastern European states, this essay argues that NATO’s successful eastern expansion has been due to the EU’s encouragement of these states’ NATO accession, not because of their enthusiasm for membership in the union. This was the case in Montenegro. Despite the country’s lack of external threats and persistent domestic opposition to NATO membership, the government nevertheless refocused its efforts from European integration to Euro-Atlantic integration in order to bring the country closer to EU accession. This codependent enlargement has benefited both organizations: the EU is able to secure its eastern boundaries without developing a comprehensive security and defense policy, while NATO is able to expand its influence over important geopolitical locations despite domestic resistance to such expansion.