The Rubicon

By December 50 BC, Julius Caesar's political position was intolerable. He had governed Gaul for eight years, killed a million people, enslaved another million, and returned to Rome as the most celebrated conqueror of his generation — and the Senate's faction, led by Cato and Pompey, intended to prosecute him for unauthorized acts the moment his military command expired. A convicted man could not hold office. Without office, he had no protection. Without protection, he was certain to be tried, and almost as certain to be convicted. Caesar's choice was between surrender and war, and Julius Caesar had not spent eight years in Gaul learning how to surrender.

On January 10, 49 BC, Caesar gathered his officers and crossed the Rubicon — the small river marking the boundary between Cisalpine Gaul (his province) and Italy proper. Roman law prohibited a general from bringing an army into Italy. By crossing, Caesar committed treason. He knew it, his army knew it, and he reportedly said 'Alea iacta est' — the die is cast — as they crossed. The statement was theatrical, but the moment it described was real: from this point forward, there was no political solution. One man or the other would die.

The speed of what followed astonished Rome. Pompey had been confident that Caesar would never actually cross, and then confident that Caesar's single legion could be contained while Pompey gathered his forces. Neither calculation proved correct. Town after town in Italy opened its gates to Caesar — not because they feared him but because his veterans inspired awe and his reputation for clemency (he released prisoners and sent them home unharmed) made resistance seem pointless. Pompey and the Senate fled to Greece, taking two understrength legions. In a few weeks, Caesar controlled all of Italy without a major battle. He then marched to Spain, where Pompey's generals held seven legions — an 'army without a general,' Caesar called it, since the general was in Greece. He defeated them in six weeks. The war was barely underway and Caesar had already won the western half of it.

The real war was in Greece, where Pompey had spent the winter training an army of nine legions, backed by the resources of the entire East. His generals held the Aegean fleet. Caesar had to cross the Adriatic against naval superiority with half his forces — a gamble that could have ended the civil war in January 48 BC if Pompey had pressed his advantage. But Pompey's strategic caution, which had served him well for thirty years, would prove fatal against an opponent for whom every battle was an existential crisis.