During the Roman Republic, the river Rubicon marked the boundary between the Roman province of Cisalpine Gaul to the north-west and Italy proper (controlled directly by Rome and its allies) to the south.
Exercising imperium when forbidden by the law was a capital offence. Furthermore, obeying the commands of a general who did not legally possess imperium was a capital offence. If a general entered Italy whilst 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.
In 49 BC, perhaps on January 10, Julius Caesar led a single legion, 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. Suetonius depicts Caesar as undecided as he approached the river, and attributes the crossing to a supernatural apparition.
According to Suetonius, Caesar uttered the famous phrase “alea iacta est” (“the dice has been cast”). Caesar’s decision for swift action forced Pompey, the lawful consuls and a large part of the Roman Senate to flee Rome in fear. Caesar’s subsequent victory in Caesar’s Civil War ensured that punishment for the infraction would never be rendered.