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Julius Caesar's crossing the Rubicon river on 10 January, 49 BC precipitated the Roman Civil War, which ultimately led to Caesar's becoming dictator for life (dictator perpetuo) and the rise of the imperial era of Rome. Caesar had been appointed to a governorship over a region that ranged from southern Gaul to Illyricum (but not Italy). As his term of governorship ended, the Roman Senate ordered Caesar to disband his army and return to Rome. He was explicitly ordered not to take his army across the Rubicon river, which was at that time a northern boundary of Italy. In January 49 BC, Caesar brought the 13th legion across the river, which the Roman government considered insurrection, treason, and a declaration of war on the Roman Senate. According to some authors, he uttered the phrase alea iacta est—the die is cast—as his army marched through the shallow river.
Today, the phrase crossing the Rubicon is a metaphor that means to pass a point of no return.
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.
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.
In January 49 BC C. Julius Caesar led a single legion, Legio XIII, south over the Rubicon from Cisalpine Gaul to Italy to make his way to Rome. In doing so, he deliberately broke the law on imperium and made armed conflict inevitable. Roman historian Suetonius depicts Caesar as undecided as he approached the river and attributes the crossing to a supernatural apparition. It was reported that Caesar dined with Sallust, Hirtius, Oppius, Lucius Balbus and Sulpicus Rufus on the night after his famous crossing into Italy on 10 January.
According to Suetonius, Caesar uttered the famous phrase ālea iacta est ("the die has been cast"). The phrase "crossing the Rubicon" has survived to refer to any individual or group committing itself irrevocably to a risky or revolutionary course of action, similar to the modern phrase "passing the point of no return". Caesar's decision for swift action forced Pompey, the consuls and a large part of the Roman Senate to flee Rome.