Russian Annexation of Crimea

In the process that unfolded following the political crisis and change of government in Ukraine, after pro-Russian forces seized control of the peninsula, a referendum that was not recognized by the greater part of the international community was held, and Crimea declared its accession to Russia. The annexation was not recognized by many countries, foremost among them Ukraine; it led to sanctions against Russia and to deep tensions in the European security order.

Source: Wikipedia