 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 Norway 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.