As a result of the 1654 Treaty of Pereyaslav, the geopolitical map of the region changed. Russia entered the scene, and the Cossacks' former allies, the Tatars, had switched sides and gone over to the Polish side, initiating warfare against Khmelnytsky and his forces. Tatar raids depopulated whole areas of Sich. Cossacks, aided by the Tsar's army, took revenge on Polish possessions in Belarus, and in the spring of 1654, the Cossacks drove the Poles from much of the country. Sweden entered the mêlée. Old adversaries of both Poland and Russia, they occupied a share of Lithuania before the Russians could get there.