During the late Roman republic, the river Rubicon marked the boundary between the Roman province of Cisalpine Gaul to the north-east and Italy proper (controlled directly by Rome and its allies) to the south. On the north-western side, the border was marked by the river Arno, a much wider and more important waterway, which flows westward from the Apennine Mountains (its source is not far from the Rubicon's source) into the Tyrrhenian Sea.