During the Roman republic, the river Rubicon marked the boundary between the Roman province of Cisalpine Gaul to the northeast and Italy proper, controlled directly by Rome and its socii (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 (it and the Rubicon rise not far from each other) into the Tyrrhenian Sea.
Governors of Roman provinces were appointed promagistrates with imperium (roughly, "right to command") in their province(s). The governor would then serve as the general of the Roman army within the territory of his province(s). Roman law specified that only the elected magistrates (consuls and praetors) could hold imperium within Italy. Any promagistrate who entered Italy at the head of his troops forfeited his imperium and was therefore no longer legally allowed to command troops.
Exercising imperium when forbidden by the law was a capital offense. Furthermore, obeying the commands of a general who did not legally possess imperium was also a capital offense. If a general entered Italy while exercising command of an army, both the general and his soldiers became outlaws and were automatically condemned to death. Generals were thus obliged to disband their armies before entering Italy.