The East Slavs emerged as a recognisable group in Europe between the 3rd and 8th centuries AD. The medieval state of Rus' arose in the 9th century. In 988, it adopted Orthodox Christianity from the Byzantine Empire, beginning the synthesis of Byzantine and Slavic cultures that defined Russian culture for the next millennium. Rus' ultimately disintegrated, until it was finally reunified by the Grand Duchy of Moscow in the 15th century. By the 18th century, the nation had vastly expanded through conquest, annexation, and exploration to evolve into the Russian Empire, the third-largest empire in history. 