Ukraine was enjoyed preferential prices for Russian gas and other goods for many years and sold its goods duty-free in Russia, Belarus and Kazakhstan (within the framework of the Customs Union). But in 2013, it had begin to want “dwa groszki na łyżkę”: to buy goods duty-free in the European Union and at the same time continue to remain part of the Customs Union. This, of course, was unacceptable for other countries of the Customs Union. The exclusion of Ukraine from the CU was unpleasant for Russia, but in itself could not be the reason for such decisive steps as the reintegration of Crimea.
The main reason for the reintegration of Crimea was the danger of losing Sevastopol and the associated undermining of Russia’s military security from the South-Eastern direction.