Crimean Peninsula which once used to be the part of USSR was occupied by the Pro-Russian forces in February 2014. Russia blamed America and EU for ousting the government of President Viktor in the wake of Ukrainian Revolution which was erupted when a series of violent events took place in Kiev - the Capital city - and the rising public pressure compelled the president to flee thus, formulating an interim government.
It was the pledge of Viktor to make certain agreements with EU in order to stabilize the country’s declining economy but in the eleventh hour he refused to deal with EU and made a pact with Russia. This act of the President sparked agitation among the masses. Later, Russia, unlike EU and USA, refused to accept the interim government and utilizing the pro-Russian factor in Crimea captured the peninsula and after the referendum was conducted, 96.77% of the Crimean people consolidated their desire to join Russia. As a reaction from the International Community, EU and America sanctioned Russia.
Till today, the Russian act of aggression is being condemned. This is a major reason for the contemporary rift between the Russian and Western government. It's not over yet. Still, Russia is interfering in Ukraine even after having dismembered it.