Sharing linguistic and cultural roots with Latvia, Lithuania's early history was significantly different. It was successful in fending off invading knights and maintaining the country's sovereignty and religion. In the 1300s, Lithuania's empire stretched to Belarus and the Ukraine. When prince of Lithuania married a Polish princess in 1386, the Lithuanian was made King of Poland on the condition that his country accept Christianity. By the end of the 18th century, Lithuania had been subsumed into Russia. Lithuanians supported the anti-Russian rebellions that arose in 1830 and 1863. The Russians responded by trying to erase the Lithuanian national identity but the people resisted and preserved a strong (if somewhat hidden) identity based on ethnicity, religion, and language. At the dawn of the second World War, the Soviets invaded Lithuania and shortly thereafter a Soviet Socialist Republic was declared. Though thousands fled (or were taken), the Lithuanians managed to thwart some of Russia's aims by preventing 'Russianization' of the region by keeping up the birth rate. As communism began to falter in the Soviet Union, Lithuania made its move, declaring its independence in 1990. The Russians resisted this action first through an economic blockage and later, by sending in troops and tanks. In September 1991, the USSR conceded and recognized Lithuania as an independent country.