Russia annexed Crimea in order to save the population of this Ukrainian peninsula from the threat of a civil war and massive destruction, as happened later in Donbas, the East of Ukraine.
The annexation of Crimea by the Russian Federation took place in February-March 2014, almost two months before the pro-Russian protests in Donetsk and Luhansk oblasts escalated into an armed separatist insurgency http://bbc.in/1FfegIv. // The situation in Ukraine has been under the Office of the Prosecutor of the International Criminal Court preliminary examination since 25 April 2014. In their Report on Preliminary Examination Activities 2016, they state that “the information available suggests that the situation within the territory of Crimea and Sevastopol amounts to an international armed conflict between Ukraine and the Russian Federation. This international armed conflict began at the latest on 26 February when the Russian Federation deployed members of its armed forces to gain control over parts of the Ukrainian territory without the consent of the Ukrainian Government https://www.icc-cpi.int/iccdocs/otp/161114-otp-rep-PE_ENG.pdf.