The East Slavs emerged as a recognisable group in Europe between the 3rd and 8th centuries AD. Founded and ruled by a Varangian warrior elite and their descendants, 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 into a number of smaller states, 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, which was the third largest empire in history, stretching from Poland on the west to Alaska on the east. Following the Russian Revolution, the Russian Soviet Federative Socialist Republic (Russian SFSR) became the largest and leading constituent of the Union of Soviet Socialist Republics (USSR/Soviet Union), the world's first constitutionally socialist state. The Soviet Union played a decisive role in the Allied victory in World War II, and emerged as a recognized 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 humans in space. Following the dissolution of the Soviet Union in 1991, the Russian SFSR reconstituted itself as the Russian Federation and is recognised as the continuing legal personality and a successor of the USSR.