The ruler of Russia during its greatest territorial expansion was Peter the Great, who reigned from 1682 to 1725. He is credited with modernizing Russia and transforming it into a major European power. Under his leadership, Russia expanded its territories significantly by waging wars against Sweden and the Ottoman Empire and gaining access to the Baltic Sea and parts of the Black Sea. Peter's reforms included military, administrative, and cultural changes, which helped to establish Russia as a formidable empire and expanded its influence across Europe and Asia.