The Russian Empire as a state, existed from 1721 until it was declared a republic 1 September 1917. The Russian Empire was the successor to the Tsardom of Russia and the predecessor of the Soviet Union. It was one of the largest empires in world history, surpassed in landmass only by the British and Mongolian empires: at one point in 1866, it stretched from eastern Europe across Asia and into North America.