Crossing the Rubicon

Caesar had previously been appointed governor of a region that stretched from southern Gaul to Illyricum. As his term was coming to an end, the Senate ordered him to disband his army and return to Rome. Caesar defied the order, and instead brought his army to Rome, occupying the city of Ariminum then crossing the Rubicon towards the south. The exact date of the crossing is unknown, but scholars usually place it on the night of 10 and 11 January because of the speeds at which messengers could travel at that time. The phrase alea iacta est ("the die is cast"), allegedly uttered by Caesar just before the crossing, also comes from this event.

History

During the late Roman Republic, the river Rubicon was a small river that flowed east from the Apennine Mountains into the Adriatic Sea. It was one of two rivers that marked the boundary between the Roman province of Cisalpine Gaul to the north and areas controlled directly by Rome and its allies to the south; west of the Apennines, the border was marked by the river Arno, a much wider and more important waterway which flows into the Tyrrhenian Sea.

Governors of Roman provinces were appointed promagistrates with imperium (roughly, "right to command") in one or more provinces. The governors then served as generals of the Roman army within the territory they ruled. Roman law specified that only the elected magistrates (consuls and praetors) could hold imperium within Italy. Any magistrate 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 a capital offense. If a general entered Italy in 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.