This happened decades ago. The area of Crimea Russia is claiming is predominantly populated by ethnic Russians. The population was established there back in the soviet block days and ethnic Ukrainians were pushed out. In more recent times these ethnic Russians did not want to be a part of the Ukraine as they were poorly represented by the current Ukrainian political class. They were essentially trying to break away from Ukraine and Russia was already leasing a naval base on the black sea from them and the Russians were essentially there at their invitation. Thats when tensions flared during the Obama administration with the US attempted Ukrainian spring back in 2014.