The main argument of this dissertation is that the interaction between the Ukraine and Russia generates a social identity of enmity, which is the source of the conflict of interests between the two countries. In order to defend the argument, a theoretical model is proposed based on the importance of ideas to the constitution of interests and on the belief that interests are determined by identities. The next task is to demonstrate why the relationship between the two countries is one of enmity. Russia does not admit the independence of Ukraine, which reacts accusing Russia of imperial mentality. Russia accepts this conferred role when it manifests its claim over the Ukrainian territory. As a result, Ukraine assures its possession of nuclear weapons to defend itself from a potential Russian aggression. By the end of the process, an enemy`s identity is already constructed. The reaction of both countries towards NATO`s expansion is used to illustrate the consequences of enmity. Because identities determine interests, the relationship between friend States involves common interests, whereas that between foes involves divergent interests. The perception of a threat is shared by friends. Between enemies, the friend of one becomes the enemy of the other. That is why Ukraine cooperates with NATO and Russia does not accept its expansion. In order to avoid that the conflicts between Ukraine and Russia become a threat to Europe`s security, it is necessary to change the identity constructed in through their interaction.