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 greatly expanded through conquest, annexation, and exploration to become the Russian Empire, the third-largest empire in history. Following the Russian Revolution, the Russian SFSR became the largest and leading constituent of the Soviet Union, the world's first constitutionally socialist state, which had a one-party system throughout most of its existence. The Soviet Union played a decisive role in the Allied victory in World War II, and emerged as a superpower and rival to the United States during the Cold War. The Soviet era saw some of the most significant technological achievements of the 20th century, including the world's first human-made satellite and the launching of the first human in space. Following the dissolution of the Soviet Union in 1991, the Russian SFSR reconstituted itself as the Russian Federation. In the aftermath of the constitutional crisis of 1993, a new constitution was adopted, and Russia has since been governed as a federal semi-presidential republic. Vladimir Putin has dominated Russia's political system since 2000, and his government has been accused of authoritarianism, numerous human rights abuses, and corruption.