Russia’s policy towards Ukraine did not simply change the system of international relations, rebuilding the relations between its key elements. Moscow’s actions in Crimea and eastern Ukraine have urged the need for sub-regional cooperation. Regionalization is one of the key trends in international relations. The author examines factors that stimulated sub-regional cooperation on the European continent (Central and Eastern Europe, South-Eastern Europe, the Black Sea region), the impact of the current global and regional processes on regional interaction, as well as the question of whether the Russian-Ukrainian crisis has raised the issue of a sub-regional security system formation due to a sharper sense of insecurity among states of CEE, SEE and the Black Sea region The factors that facilitate or hamper efficiency of regional cooperation are studied as well.