An important river in American history, the Rappahannock was long an area of occupation by indigenous peoples. During the colonial era, early settlements in the Virginia Colony were formed along the river. It was at the center of a major theater of battle in the American Civil War, where tens of thousands of troops fought against each other. Some 10,000 African-American slaves escaped across the river to Union lines and freedom, after the first Battle of Fredericksburg. Due to the river's significance as an obstacle to north-south movements, it effectively functioned as the boundary of the eastern theater of the war, between the "North" (the Union) and the "South" (the Confederate States of America).