After the 13th century, Moscow became a cultural center. By the 18th century, the Tsardom of Russia had become the huge Russian Empire, stretching from the Polish–Lithuanian Commonwealth eastward to the Pacific Ocean. Expansion in the western direction sharpened Russia's awareness of its separation from much of the rest of Europe and shattered the isolation in which the initial stages of expansion had occurred. Successive regimes of the 19th century responded to such pressures with a combination of halfhearted reform and repression. Russian serfdom was abolished in 1861, but its abolition was achieved on terms unfavorable to the peasants and served to increase revolutionary pressures. Between the abolition of serfdom and the beginning of World War I in 1914, the Stolypin reforms, the constitution of 1906, and State Duma attempted to open and liberalize the economy and politics of Russia but the tsars were still not willing to relinquish autocratic rule or share their power.