After the 13th century, Moscow became a political and cultural center. Moscow has become a center for the unification of Russian lands. By the end of the 15th century, Moscow united the northeastern and northwestern Russian principalities, in 1480 finally overthrew the Mongol yoke. The territories of the Grand Duchy of Moscow became the Tsardom of Russia in 1547. In 1721 Tsar Peter the Great renamed his state as the Russian Empire, hoping to associate it with historical and cultural achievements of ancient Rus' – in contrast to his policies oriented towards Western Europe. The state now extended from the eastern borders of the Polish–Lithuanian Commonwealth to the Pacific Ocean. Russia was a great power and dominated Europe after the victory over Napoleon. Peasant revolts were common, and all were fiercely suppressed. The Emperor Alexander II abolished Russian serfdom in 1861, but the peasants fared poorly and revolutionary pressures grew. In the following decades, reform efforts such as the Stolypin reforms of 1906–1914, the constitution of 1906, and the State Duma (1906–1917) attempted to open and liberalize the economy and political system, but the Emperor refused to relinquish autocratic rule and resisted sharing his power.