Caesar had been appointed to a governorship over a region that ranged from southern Gaul to Illyricum. As his term of governorship ended, the Senate ordered Caesar to disband his army and return to Rome.

In January 49 BCE 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.

Caesar's most trusted lieutenant in Gaul, Titus Labienus defected from Caesar to Pompey, possibly due to Caesar's hoarding of military glories or an earlier loyalty to Pompey.

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. Julius Caesar's crossing of the river precipitated the Great Roman Civil War.