Complex sentence: The Russian-controlled authorities held a referendum on Crimea joining the Russian Federation, after which the territory was formally joined to the Russian Federation as two federal subjects: the Republic of Crimea and the federal city of Sevastopol.
Split into simpler sentences: The Russian-controlled authorities held a referendum on Crimea joining the Russian Federation, after which the Russian government proclaimed official control over the peninsula. Russia currently administers the peninsula as two federal subjects: the Republic of Crimea and the federal city of Sevastopol.