By the spring of 49 BC, the hardened legions of Caesar crossed the river Rubicon, the legal boundary of Roman Italy beyond which no commander might bring his army, and swept down the Italian peninsula towards Rome, while Pompey ordered the abandonment of Rome. Afterwards Caesar turned his attention to the Pompeian stronghold of Hispania (modern Spain) but decided to tackle Pompey himself in Greece. Pompey initially defeated Caesar, but failed to follow up on the victory, and was decisively defeated at the Battle of Pharsalus in 48 BC, despite outnumbering Caesar's forces two to one, albeit with inferior quality troops. Pompey fled again, this time to Egypt, where he was murdered.

During what year did Caesar's armies cross the river Rubicon?