This article appears in the March/April 2017 issue of National Geographic History magazine.

On January 10, 49 B.C., on the banks of the Rubicon River in southern Gaul (near the modern-day city of Ravenna), Julius Caesar and the soldiers of the 13th Legion waited and weighed their options.

The Rubicon is, in reality, little more than a stream. Its significance to Rome lay in its location, marking the official border between Italy and Cisalpine Gaul, the region south of the Alps governed by Julius Caesar. Despite its appearance, crossing this humble river would have serious consequences. According to the law of the Roman Republic, any provincial governor leading troops across the border back into Italy would be declared a public enemy. It was, quite simply, an act of war.

Huddled against the biting cold, many of the soldiers of the 13th Legion of the army of the Roman Republic had served under Caesar for much of the previous decade. They had witnessed the honing of his skills as a military and political strategist, subjugating Gaul (corresponding to much of modern-day France and northern Italy), extending the bounds of the Roman Republic as far as the Rhine, and all the time shoring up his influence back in Rome. Alarmed by his growing power, the Senate ordered Caesar to set aside his command.

Caesar had no intention of obeying the Senate, and he knew perfectly well what the consequences of his insubordination would be. He understood that civil war would most likely ensue between himself and the Roman nobility, led by his strongest rival and former ally: the brilliant military commander Pompey the Great. If Caesar chose to cross the Rubicon, there would be no turning back.

Down to the River

The day before the crossing, Caesar acted as if nothing unusual was happening. The conqueror of Gaul attended a public event in Ravenna and carefully examined plans for a gladiator school. Secretly, he had ordered his cohorts to proceed to the banks of the river and wait for him there. Later, during dinner that night, he told his guests he would have to leave them for a moment. A chariot pulled by mules from a nearby bakery was waiting for him outside, and after a considerable delay in finding the exact position of his troops, he eventually managed to join them on the bank. Here he mulled the agonizing choice that lay before him.

Writing around a century and a half later, the historian Suetonius produced an account of this moment that reveals the legendary status the event had attained in the Roman mind. Still unsure whether to advance, a man of extraordinary height and beauty appeared, clearly sent by the gods. “The apparition snatched a trumpet from one of them, rushed to the river, and sounding the war-note with mighty blast, strode to the opposite bank. Then Caesar cried: ‘Take we the course which the signs of the gods and the false dealing of our foes point out. The die is cast.’”

View Images Roman Bank Heist Rome’s treasury was housed in the Temple of Saturn at the Forum; it was sacked by Caesar in 49 B.C. to pay for his wars against Pompey. Photograph by Alessandro Saffo/Fototeca

The Path to Power

Caesar was not the first person to openly violate the law of the republic. Social tensions created by the rapid expansion of Roman territory had plunged the political system into crisis for much of Caesar’s life. During his youth, generals and politicians often exploited their military victories to take political control of the state.

Born around 100 B.C., Caesar’s boyhood was marked by the Social Wars, a series of struggles in which Rome’s Italian allies fought for the right to Roman citizenship and its privileges. In 81 B.C. Sulla was appointed dictator. Sulla defended the rights of Rome’s increasingly discredited noble rulers against the populares, the Senate faction who represented the interests of non-noble citizens clamoring for reform.

Caesar’s career was marked by this atmosphere of frenzied competition for power between nobles and populists. In 60 B.C. he allied with the general Pompey and another powerful politician, Marcus Licinius Crassus, so the three of them could dominate the republican system for their own benefit. The immediate result was Caesar’s consulship in 59 B.C., during which he sidelined the Senate and passed various laws aimed at winning him popular support.

View Images The Gaul to Oppose Caesar France’s capital’s name comes from the wealthy Parisii tribe, against whom Caesar fought in Gaul. The Parisii minted this second-century B.C. gold coin. Photograph by Erich Lessing/Album

Stung by Caesar’s affronts, the powerful aristocratic faction in the Senate—known as the optimates—were waiting to pounce on him when his consulship ended, when he would be left without official immunity and highly vulnerable to his enemies. Revealing his lifelong instinct for survival, however, Caesar cut a deal with Pompey and Crassus, enabling him to leave for Gaul to achieve the military glory that would, in turn, increase his grip on power.

Eight years later, at the beginning of the year 50 B.C., Caesar had subjugated Gaul, to the great benefit of the republic, which had won valuable territory to defend it against invasions. But the main beneficiary of the wars was undoubtedly Caesar himself. Awash with Gallic gold, he shrewdly targeted financially embarrassed senators who, in return for Caesar’s “generosity” in paying off their debts, declared themselves his unconditional allies. At the same time, he had at his back a trained, experienced, and fiercely loyal army. Caesar’s combination of wealth and military clout struck fear and loathing into the hearts of senators back in Rome—not least his erstwhile ally, Pompey, who since Crassus’s death had been moving politically closer to the aristocratic optimates.

After the fighting was ended in Gaul, Caesar was obliged to stand down from his position as governor, disband his army, and so lose the immunity his official position had given him. Pompey and his new optimate allies hatched a plan to seize the moment to take Caesar to court. By accusing him of corruption and abuses of power during his time in Gaul, they hoped to bring his political career to an end.

But Caesar stood his ground in March 50 B.C. He would not stand down as governor of Gaul, as stipulated, but would instead stay on until the end of 49 B.C., proposing that in the summer of that year, he would stand for election to become consul for a second time.

Faced with such obstinacy, his enemies in Rome scrambled to increase the pressure on the rogue governor. They reiterated to the Senate that since the military campaign was over, Caesar must disband his army, and a new governor of Gaul be elected to replace him.

The hostile atmosphere in the Senate convinced Caesar that he needed to defend himself militarily and politically. He moved some of his troops into the north of Italy, at the same time extending his influence in the corridors of power. Bribery continued to be the most effective tool. In a particularly spectacular coup, he even managed to buy off the consul Lucius Aemilius Lepidus Paullus for a colossal down payment of some nine million denarii. In return, the consul promised not to support any initiatives against him during his remaining term of office.

Pompey Versus Caesar

View Images Gnaeus Pompeius Magnus, “Pompey the Great.” 18th-century bust. Museum of Roman Civilization, Rome Photograph by Prisma Archivo

The deadline for Caesar to lay down his command, March 50 B.C., came and went. An emergency briefly offered a way out of the stalemate: The Parthians were threatening Rome’s eastern borders and the Senate was asked to send two legions to defend the province of Syria. Pompey declared that he would send one if Caesar sent another. Caesar surprisingly accepted, perhaps to demonstrate a willingness to compromise. According to Caesar’s own account of these tumultuous years, The Gallic Wars, the legion assigned to Pompey was Caesar’s anyway. Keeping his promise, Caesar handed over the 15th Legion, quartered in Cisalpine Gaul, only to find out that the Parthian threat had petered out and that both legions now lay firmly under the control of Pompey in Italy.

Far from undermining Caesar’s confidence, Pompey’s deceitful maneuver only seemed to stiffen his resolve. Throughout that year, the brinkmanship between the two generals grew, and nerves stretched to breaking point. A false rumor spread that Caesar had set out from Gaul with four legions. The statesman and orator Cicero vainly tried to find a peaceful solution to the conflict while a sense that the republic was becoming increasingly ungovernable took hold in the capital. Alliances shifted continually: One of Caesar’s most loyal lieutenants, Labienus, decided to switch sides to Pompey.

View Images The Great Orator A bust from the Capitoline Museums in Rome depicts Cicero, an ally of Pompey, who later developed a cordial relationship with Caesar. Photograph by Prisma Archivo

Meanwhile, Pompey had convinced himself that his forces were stronger than Caesar’s and that his charismatic leadership would enable him to recruit as many men as he wished in Italy. According to the late first-century historian Plutarch, a contemporary of Suetonius: “When they said that if Caesar was heading to Rome they could not see what troops could withstand him, Pompey boastfully replied with a smile: ‘Legions will spring up anywhere I stamp on the ground in Italy.’”

Marcus Caelius Rufus, an aristocrat, summarized the situation in a letter to Cicero in the fall of that year: “The closer we come to this inevitable clash, the more apparent the danger. At the heart of the issue is this: Pompey declares he won’t allow Caesar to be elected consul unless Caesar relinquishes control over his army and provinces; Caesar, on the other hand, is convinced his status is threatened if he gives up his troops ... So now ... their scandalous liaison isn’t stepping behind the scenes ... but exploding into full-scale war!”

Most of the terrified senators were willing to grant the concessions Caesar was asking for to avoid war. In December, when the plebeian leader Curio persuaded the Senate to vote on the proposal for Caesar and Pompey to lay down their arms at the same time, 370 senators voted for it and just 22 against. But the faction opposing Caesar immediately went against the spirit of this decision. They sought out Pompey in the Forum and dramatically placed a sword in his hand, begging him to take command of Italy’s troops to save the republic. They urged him to take command of the army and of as many additional troops as he wanted to recruit himself. Although he was breaking the law, Pompey accepted the mission.

Element of Surprise reveal Continue Tour 1 of many : Caesar’s risky move in crossing the Rubicon surprised his opponents. Pompey judged it unwise to fight Caesar in Italy and decided to withdraw to Greece and the east, where he was able to recruit a well-trained army. His allies fled Italian towns and cities as Caesar approached. Lucius Domitius Ahenobarbus was the only one who fought back. He dug in at Corfinium. But his own men mutinied and handed him over to his enemy together with other senators. Caesar let them go free, knowing full well they would go back to Pompey—which they did. This magnanimous and unexpected gesture was calculated to show the public that he was no tyrant, but rather a man on the side of the people and the republic, an image he would foster until the end of his life. In this map, the white line shows Caesar’s invasion route through Italy in 49 B.C., while the green line shows Pompey’s route. Black lines indicate detours made by Caesar’s troops. After crossing the Rubicon into the Roman Republic near Ravenna in early January, Caesar marches his forces south to Ariminum. Pompey and his allies abandon Rome for Capua. Caesar sends the tribune Mark Antony with five cohorts of the 13th Legion to occupy Arretium. He sends another three cohorts to the coastal cities of Pisaurum, Fanum Fortunae, and Ancona. Caesar advances south, easily taking the cities of Pisaurum, Fanum, and Ancona. Some of his troops are dispatched west to the city of Iguvium; these forces easily take the city. Cingulum opens its gates to Caesar’s troops without any bloodshed. Pompey’s garrison at Asculum leaves their post before the arrival of Caesar’s forces as they move down the coastline. Caesar makes his camp in Apulia. His troops approach Sulmo, which immediately surrenders. Corfinium falls with the capture of three legions. Many of these soldiers join up with Caesar. As Caesar advances with an ever growing army, Pompey abandons Capua for Brundisium. Caesar races to cut him off, but Pompey sails to the Greek shore with his troops in mid-March. MAP: EOSGIS.COM

View Images Divine Intervention Suetonius tells of a miraculous event that occured as Caesar mulled over crossing the Rubicon, depicted in this 1494 painting by Francesco Granacci. As Caesar debates whether to cross the Rubicon, an otherwordly figure appears, wearing a yellow tunic and playing a lute (left). Caesar and his soldiers follow the figure (left of center). Caesar believes the gods are on his side, encouraging him to proceed into Italy. The mysterious being snatches a trumpet from a Roman soldier and plays it as he crosses the river, followed by Caesar’s army. On reaching the other bank, Caesar (in a blue toga) inspires his soldiers with a speech before continuing their march (right of center). The plebeian tribunes supporting Caesar surround him, aware of the momentousness of the occasion, and plan their next movements. Caesar leads his army to Rome, shown here surrounded by strong walls (right), to take what is his by force. Photograph by Vanda Images/Photoaisa

The Moment of Fate

As the year 49 B.C. opened, Caesar sent the Senate a letter from Ravenna, giving them his final word on the matter. He again offered to resign his command at the same time as Pompey, but the Senate interpreted his proposal as a gesture of arrogance. Pompey and the consuls prevented a vote on the proposal in the letter and passed a motion declaring Caesar a public enemy. The motion was vetoed by Mark Antony, the newly appointed plebeian tribune and crucial ally of Caesar who would prove to play a fateful role in the last stages of his life. Even so, negotiations went on until the very last moment. Caesar even said he would stand down if he were allowed to keep just one legion and govern the province of Illyria, in the modern-day Balkans. The proposal might have been acceptable but was rejected due to fierce opposition by Cato the Younger, one of Caesar’s most implacable opponents.

View Images Roman Legionnaire bronze statue, second to first centuries B.C., Museum of Roman Civilization, Rome Photograph by Art Archive

The Senate met again and passed a decree calling on the consuls to defend Rome against any attack. The tribunes Mark Antony and Quintus Cassius (a relative of the famous Cassius who later conspired to assassinate Caesar) exercised their veto, but it was rejected by the Senate. Fearing for their lives, Mark Antony and Cassius fled Rome disguised as slaves and joined Caesar in the north.

Writing later in The Civil Wars, Caesar recalled how he had been waiting for the Senate’s response for days “[to see] if matters could be brought to a peaceful end by any equitable act on the part of his enemies.” But he now realized there was no other way and started preparing for the final showdown. Around January 10, when he learned of the Senate’s decision, he ordered the 13th Legion to take up their riverside positions, exhorting them to defend the honor of their general whom they had served for nine years. They in turn swore to avenge the insults against him and the tribunes.

Caesar now had the backing of a loyal army who would follow him to victory or death. According to the poet Lucan, Caesar declared: “Here I abandon peace and desecrated law. Fortune, it is you I follow. Farewell to treaties. From now on, war is our judge.”

View Images A Crowning Moment In 44 B.C. Mark Antony presented Caesar a crown at Rome’s Temple of Castor and Pollux. Despite rejecting Antony’s offer, Caesar was assassinated a month later. Photograph by Angelo Cavalli, Age Fotostock

After the Crossing

The choice facing Rome was either decades of more factionalism and political chaos, or accepting a strongman to impose reform, and set its affairs in order. On swiftly passing to the far bank of this minor river, Caesar set the republic hurtling down the second course.

Since crossing the Rubicon, Caesar and his legions had expelled Pompey and his troops from Italy. But this campaign was just the beginning. In the attempt to destroy Pompey and his extensive allies across the Roman world, Caesar was forced to cover astonishing distances, putting down a revolt in modern-day Marseille in France before routing Pompey’s loyalists in Spain at the Battle of Ilerda in June.

The following year, 48 B.C., Caesar dedicated to pursuing Pompey across Greece. Crossing to Egypt after his defeat at the Battle of Pharsalus, Pompey threw himself on the mercy of Ptolemy XIII, who immediately had him murdered. The Egyptian ruler evidently saw where the tide of Roman power was flowing.

Shortly after the removal of his military rival, Caesar arrived in Egypt, where he patched up the dynastic struggle between Ptolemy and his sister, Cleopatra VII. Little knowing she would become the last of the Ptolemaic monarchs who had ruled Egypt since the time of Alexander the Great, Cleopatra spent much of that same year as Caesar’s lover, sealing her reputation in the Roman world as a femme fatale who would later “ensnare” Mark Antony.

View Images A Key Ally Depicted on the back of an aureus minted in Gaul, Mark Antony was entrusted with the command of the left wing of Caesar’s army in major battles. Photograph by AKG/Album

As the Alexandrian romance eventually faltered, war once again proved the constant in Caesar’s life. Despite the death of their captain, Pompey’s supporters had regrouped in order to avenge him, and Caesar was forced to buckle on his armor again, briefly returning to Rome before dealing a crushing blow against his enemies in modern-day Tunisia in 46 B.C. Even then, resistance to the new order bubbled up in the following months. Caesar only delivered the death blow to Pompey’s stubborn followers in Spain a year later, in 45 B.C.

Having returned to Rome, he continued implementing significant reforms in the year of life left to him. These included improving land and grain distribution, as well as the reorganization of local government across Italy. No doubt Caesar hoped for many years of life to enact his reforms—but where he had defeated his enemies on the battlefield, he proved more vulnerable in the corridors of power. On that infamous March day the following year, he succumbed to the assassins’ knives.

What would have happened had Caesar not made that fateful river crossing five years previously? What would have happened if the republican old guard that assassinated him had prevailed over Mark Antony, reinforced republican power, and steered the Roman world away from autocratic rule?

In the event, he was succeeded by his adopted son, Octavian, who consolidated the drift toward authoritarian leadership, later becoming the Emperor Augustus. Under this brilliant, implacable leader, the new Roman Empire buried the old, aristocratic infighting to become a global power, whose astonishing legacy continues to shape the modern world.