Why Do We Say ‘Crossing the Rubicon’?

In January 49 B.C., Julius Caesar took a small step across the Rubicon River that would reshape the trajectory of Ancient Rome. Although the threshold was easy to traverse, Roman law decreed if any army crossed the boundary, it was an act of war.

As a general for the Roman Republic, Caesar had led an eight-year campaign to conquer Gaul (parts of modern-day France, Belgium, Switzerland, Germany and Holland). He earned a fierce reputation, as well as the loyalty and respect of his soldiers. But Pompey—Rome’s leading general and statesman—grew jealous. He and his allies in the Roman Senate told Caesar to demobilize and return to Rome without protection. 

Caesar’s refusal didn’t just kick-start a civil war; it was a turning point for the nearly 500-year-old Roman Republic—where elected officials had shared power—that ultimately led to an autocratic dictatorship.

Born into an aristocratic family of modest means in July 100 B.C., Julius Caesar rose through the political and military ranks. He formed an alliance with Pompey and wealthy statesman Crassus for political sway in Rome. Caesar left in 58 B.C. to become the governor of Gaul and Illyricum, during which time he expanded the Roman Republic’s boundaries substantially with the Gallic Wars.

“He advances all the way to the English Channel, the Atlantic Ocean and the Rhine River. He even invades Britain, not to take it but to show his force,” says Edward Watts, a history professor and author of The Romans: A 2,000-Year History. 

As news of Caesar’s success got back to Rome, senators and politicians “started whispering in Pompey’s ear about Caesar’s ambition and how dangerous it was,” says Watts. With the Gallic Wars coming to an end, Pompey instructed Caesar to give up his army. But Caesar didn’t want to leave himself vulnerable to “any kind of malicious prosecution,” adds Watts. “Caesar eventually counters and says he’ll only give up his army if Pompey gives up his army, which is a non-starter.”

When the Roman Senate publicly declared Caesar an enemy of the state, Caesar realized his “only option was to take matters into his own hands and use force,” says Watts. He brought 5,000 troops to the city of Ariminum (now Rimini), just a few miles north of the Rubicon River.

On January 11, 49 B.C., Caesar and his army crossed over the Rubicon River. Despite its place in history, “there’s no fighting at all. There’s not really anybody to stop Caesar from marching over the Rubicon,” explains Watts. The Roman Senate had given Pompey the power to raise his own army in defense, but they weren’t ready.

Because the attack took place during winter, other armies were “in their camps while Caesar’s were marching around," explains Josiah Osgood, author of Caesar's Legacy: Civil War and the Emergence of the Roman Empire.

Caesar then unexpectedly sent his troops south to Rome so quickly that he took the city with relative ease. “The Senate had expected Caesar to solidify his position in Gaul,” remarks Watts. “But he goes on the offensive and forces Pompey and the senators loyal to him to evacuate Italy.”

As Pompey retreated, members of the Roman Senate were forced to take sides. After taking control of the resources in Rome, Caesar used the treasury’s money to fund the impending civil war. His charisma and oratory skills also helped him build a larger following. 

By the end of 49 B.C., Pompey and his legion had been pushed out to Greece. Even though they’d abandoned Rome, Pompey claimed “they can be the government of Rome wherever they are,” says Osgood, who adds that Pompey also didn’t want to fight on Italian soil.

Caesar took the fight to Pompey. Battles took place in Spain, Greece and North Africa. After defeating Pompey’s army at the Battle of Pharsalus on August 9, 48 B.C., Pompey fled to Egypt. But Pharaoh Ptolemy XIII had Pompey assassinated before he could even step ashore. Ptolemy sought Caesar’s support in Egypt’s own civil war against his sister Cleopatra VII.

The Roman Civil War continued on until 45 B.C. By then, Caesar had defeated opponents such as Metellus Scipio, Cato the Younger and Titus Labienus.

Once Caesar crossed the Rubicon, “there was no going back to this idea in the Republic that everyone was more or less equal in the Senate,” explains Watts. “Everybody understood that a dominant figure of some sort would run Rome.”

Caesar named himself dictator for life in 44 B.C. He pardoned senators like Brutus and Cassius, who had fled with Pompey, as “these people knew how to run the state,” says Watts. He also canceled debts and redistributed land for the poor in an effort to reconcile Rome.

But Roman senators began to plot against Caesar, especially when he released coins bearing his image, turned his birthday into a public holiday and began to rule from a golden throne. “The Crossing of the Rubicon and the aftermath show them that Caesar is ultimately going to choose his own survival over the principles of Rome's Republic,” says Watts.

On March 15, 44 B.C., Brutus and Cassius led a group of 60 senators who stabbed Caesar to death. Though they wanted to save the Roman Republic, they instead created a power vacuum and spurred more civil wars. In 27 B.C., Augustus took absolute power and effectively became the emperor of Rome (although he referred to himself as Princeps, the First Citizen).  

Despite occurring over 2,075 years ago, "crossing the Rubicon" is still used as a metaphor when someone makes a decisive choice from which they can’t turn back. The phrase that Julius Caesar uttered just before he took those steps, “The die is cast,” also commonly signifies an irreversible decision.

"Caesar was exceptionally good at crafting phrases that people remember. He was trained by rhetoricians who emphasized that you waste no words,” says Watts, who highlights that the famous Latin phrase, “Veni, vidi, vici (I came, I saw, I conquered)” is also attributed to Caesar. 

Many people were “invested in Caesar’s legacy” and were intent on sharing his capacity to “distill the meaning of a particular moment concisely and powerfully" as propaganda, Watts explains. They probably “replicated that style and passed down things that Caesar either did say or things that he could have said.”