The Cossacks were escaped Russian serfs, who had establ'd 'free steppe' communities under elected military leaders. From the 15th century the Cossacks were recognized by the Tsars and used to protect the frontiers against the Turks.

Commissioned by the Stroganovs in 1581, the Cossacks advanced to the river Irtysh; in 1610 they reached the mouth of the Yenisey. From 1619 this river marked Russia's eastern border. Central Siberia was penetrated within 30 years and in 1645 the explorer Deshnev reached the Pacific Ocean. A Russian / Chinese border conflict was settled by the first Chinese - European agreement, the Treaty of Nerchinsk (1689).