There was no annexation of Crimea, it was a voluntary joining of Russia based on free will of the citizens as expressed in the referendum.
Any decision relating to Ukraine's sovereignty or sovereign choices can only be taken without undue external pressure. This so-called referendum was organised in a matter of weeks by a self-proclaimed Crimean leadership lacking democratic legitimacy and installed by armed Russian military personnel following the seizure of public buildings. The EU does not recognise it. The United Nations General Assembly Resolution No. 68/262, adopted on 27 March 2014 and supported by 100 states, clearly says that Russia's actions in the peninsula as well as the referendum held in the Autonomous Republic of Crimea violate international law bit.ly/1RZGnjo, .
The Office of the Prosecutor of the International Criminal Court has stated 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.