Julius Caesar’s incredible victory over Pompey at Pharsalus against all odds

Caesar's growing popularity, fuelled by his military success in Gaul and his generous distribution of war spoils, threatened the authority of the senatorial elite who remained wary of any single man accumulating too much influence.

Instead of complying with the Senate’s order to disband his army and return to Rome without the protection of his legions, Caesar had made a deliberate decision to cross the Rubicon River in January 49 BCE, knowing that such an action would spark a civil war.

However, he was determined to preserve both his status and his political survival. 

Pompey, who had once served alongside Caesar in the First Triumvirate, aligned himself with the Senate following Crassus’ death in 53 BCE and began to build his power as the Republic’s protector.

Influential senators such as Cato the Younger, Metellus Scipio, and Cicero viewed Caesar’s defiance as an existential threat to Republican traditions and urged Pompey to act.

Although he retained extraordinary military authority and control over the provinces, Pompey underestimated how quickly Caesar would advance on Rome and force him to retreat across the Adriatic.

Within weeks, Pompey and most of the Senate had fled to Greece, where they had begun raising an army from Rome’s eastern allies while Caesar had secured his control over Italy and had pushed into Hispania to neutralise Pompeian forces stationed there. 

After securing Spain and stabilising his western front, Caesar crossed into Epirus with a smaller force, aiming to force a confrontation before Pompey’s numerical superiority could become overwhelming.

A campaign of movements aimed at wearing down the enemy followed, and by August 48 BCE both commanders prepared for open battle near the Enipeus River in Thessaly.

The battle, which Caesar welcomed despite the odds, would determine which commander would emerge with clear military and political control, though further resistance in Africa and Hispania still awaited him. 

Pompey commanded an army of approximately 38,000 to 45,000 infantry and around 6,700 cavalry, recruited from a wide network of eastern provinces and supported by aristocrats who supplied both money and men.

His troops had access to better supplies, food, and equipment, which flowed easily through secure Greek ports and road systems.

Many of his legions had served under various commanders and possessed combat experience, though few had fought together as a unified force under Pompey’s direct leadership.

Among these were two legions that Caesar had previously lent to the Senate, which made their presence at Pharsalus a bitter irony. 

Caesar, by contrast, brought to Pharsalus an estimated 22,000 infantry and around 1,000 cavalry.

These figures are based on Caesar's own account, and included some of his most battle-hardened units, such as Legio X Equestris, which had served him faithfully throughout the Gallic campaigns.

His troops had endured a harsh march through the Balkans, faced shortages of grain, and had grown thin from hunger and exposure.

However, these men had spent years campaigning with Caesar in Gaul, and their loyalty had already been tested in sieges, ambushes, and major set-piece battles.

Their unity had been forged through hardship and repeated success, and created a force that trusted its commander implicitly and responded to orders with speed and discipline unmatched by any other army in the Roman world. 

Specifically, Pompey relied on his cavalry advantage to crush Caesar’s right flank and encircle his line.

Confident in his superiority, he ordered his infantry to hold position, assuming that Caesar’s exhausted soldiers would lose momentum and collapse under pressure.

He positioned his cavalry on his left wing under the command of Labienus, a former lieutenant of Caesar who had defected after the outbreak of civil war, and expected that the mounted charge would cause a rout before the infantry clashed in full. 

Before the battle began, Caesar had recognised the threat posed by Pompey’s large cavalry force and had prepared an unusual response.

Behind his thin right flank, he concealed a reserve line of veteran infantrymen drawn from his third line, and he instructed them to hold their pila and to wait for the moment when the enemy horsemen attacked.

When Pompey’s cavalry swept forward, Caesar’s hidden line advanced quickly and struck upward into the faces of the riders and targeted their eyes, which forced the riders into chaos and retreat.

This tactic, as later described by Plutarch, broke the cavalry's formation and caused a rapid collapse. 

The collapse of Pompey’s cavalry exposed his left flank to attack, and Caesar, seizing the opportunity, ordered his right wing to press forward.

His centre, which had advanced more slowly than Pompey expected, met the senatorial infantry on more equal footing.

The combined push from Caesar’s front and the attack on the exposed left created a break in Pompey’s formation, which Caesar’s forces widened with every metre of ground gained.

Once the senatorial troops began to fall back, Caesar pressed the advantage by ordering his reserves into the gap. 

Within a short time, panic spread among Pompey’s infantry. Soldiers broke rank and fled across the plains toward the river and surrounding hills.

Many drowned in the Enipeus or fell under the swords of Caesar’s pursuing soldiers.

Caesar, who had instructed his men to show restraint, allowed many to surrender, and he stated aloud that he still viewed them as fellow Romans.

His deliberate mercy set him apart from other commanders and secured the loyalty of many prisoners taken in the aftermath.

According to Appian, nearly 15,000 Pompeian troops were killed and over 24,000 surrendered.

However, other ancient sources present differing figures, and modern historians caution that the exact death toll is uncertain. 

Pompey was shocked by the rapid and large-scale defeat, and he fled the battlefield and made his way to Egypt.

He had believed he could rally support from the Ptolemaic court, but he instead encountered betrayal, as the young king Ptolemy XIII, engaged in a dynastic struggle with his sister Cleopatra VII, ordered his assassination in a failed attempt to win Caesar’s favour. 

Caesar excelled as a field commander who could adapt his plans under pressure and rely on personal bonds with his troops to carry out difficult manoeuvres in moments of crisis.

He moved freely among his legions, spoke directly to soldiers by name, and shared in their suffering, which earned their trust in a way that most Roman generals never achieved.

His flexible thinking, combined with years of experience from leading smaller forces against larger ones, gave him a clear advantage in fluid battlefield conditions. 

Pompey, by contrast, preferred to delegate responsibilities and relied on others to carry out his strategy.

Although he had once enjoyed great success in earlier campaigns, including victories in the East and suppression of piracy, he had not faced an opponent of Caesar’s calibre in many years.

His tendency to avoid risk and his over-reliance on numerical superiority revealed an inability to respond effectively once his initial plan faltered.

As a result, Pompey remained behind his lines, observed the battle from a hilltop, and failed to issue new orders as the situation changed rapidly. 

Another important difference lay in the armies themselves. Caesar’s troops formed a tight unit with shared purpose and years of battlefield experience under one general.

Pompey’s forces included many units drawn hastily from different provinces, some loyal to the Senate, others to specific commanders, and most lacking the unity needed to endure a sustained assault when leadership broke down.

Many of these men had never fought together before Pharsalus. 

The defeat of Pompey at Pharsalus signified the collapse of the senatorial resistance to Caesar’s authority and left the Republic without an effective military opposition.

With the enemy army destroyed and its leaders either dead or fleeing, Caesar could dictate terms to the remaining senators and alter the power structure of Rome to his advantage.

No further force existed that could match his strength or challenge his position. 

After the battle, Caesar moved swiftly to consolidate control over the eastern provinces, restore order in Egypt, and return to Rome with expanding authority.

In the years that followed, he accepted special powers and titles that accumulated over time: the Senate first named him dictator for one year in 49 BCE, later lengthened the office to ten years in 46 BCE, and by early 44 BCE effectively granted him the dictatorship for life.

Although he refrained from declaring himself king, Caesar accepted extraordinary powers and titles, including dictatorship without term limits, and used these to pass reforms and stabilise his control.

He pardoned many enemies, including Brutus and Cassius, in an attempt to restore unity.

The institutions of the Republic, weakened by years of division, no longer had the ability to resist a single ruler backed by military force. 

Historians have viewed Pharsalus as the moment when the Republic ceased to operate as a political system based in collective decision-making.

The ideals of senatorial debate and shared power lost their real power under the pressure of civil war, and Caesar’s victory at Pharsalus removed the last effective military obstacle to his supremacy, clearing the path toward monarchy.

Though his own life ended violently five years later, the changes he enforced continued through the rise of Augustus, who used the ruins of the Republic to construct a permanent imperial regime.