This article will explain why Russia annexed Crimea and is destabilizing eastern Ukraine. To do this, three different theoretical approaches on various levels of analysis will be used. It will be examined how far the expansion of NATO, as well as that of the European Union (Theory of Neorealism), was a motive for Russia’s action. NATO’s enlargement is analysed predominantly. In addition, political-psychological motivations of the Russian leadership are considered. But it is also analysed whether Russia’s pure power interests have played a role (Theory of Realism). The focus here is on the Russian naval base in Crimea. It is necessary to examine whether preserving its fleet in the Black Sea was a motive for Moscow to annex the Crimean peninsula.