Meanwhile in Rome, Roman Empire

Whilst Caesar conquers in Gaul, cracks begin to appear in the Triumvirate. In 53 BCE Crassus gains his own command. He hopes to push even further east than Pompey, attacking the Parthians (an Iranian-speaking people who had been building a rival empire to Rome) in Mesopotamia, modern-day Iraq. Their vast lands allow access to the profitable trade routes of the Far East and the breadbasket lying between the Tigris and Euphrates rivers. But there are personal reasons too: Crassus is being overshadowed by Caesar and has never reached the heights of Pompey. He wishes to strengthen his hand in an alliance where, despite his age and resources, he is quickly becoming the junior partner.

The command is ill-thought-out, and the Romans severely underestimate the Parthians. Crassus meets their general Surena at Carrhae in modern-day Turkey. Behind him, Surena has the cataphracts, the flower of the Parthian nobility, resembling medieval knights clad head to toe in shining, scaled armour, and on his wings, he has squadrons of highly mobile archers mounted on horses. Surena’s light cavalry run circles around the Roman legions, exhausting them, harassing them with volleys of arrows, making them easy prey for a massed charge of cataphracts. The Romans are destroyed, and Crassus dies, thwarted, humiliated and alone somewhere on the Anatolian steppe.

Then Julia, Caesar's daughter and Pompey's wife, dies in childbirth. With Crassus dead, there is no mediating force between the ambition of Caesar and the might of Pompey, and without Julia, their familial bond is severed. The Triumvirate is no more.

Pompey begins to move closer to the optimates faction. Men like Cato the Younger, the upright and moralistic conservative senator, Marcus Tullius Cicero, the orator and chronicler of the Late Republic, Gaius and Marcus Marcellus of the influential Marcellus clan and Quintus Metellus Scipio, descendant of Scipio Africanus, conqueror of Carthage, begin to organise a resistance. Pompey and his allies read Caesar’s triumphant and aggrandising correspondence from Gaul with growing fear and begin a political campaign against him.

In 52 BCE, Pompey assumes the sole consulship. This was extraordinary and unprecedented, with the office always being shared by two people. In the optimates’ desperate attempt to curb Caesar’s growing power, they have, for a year, made Pompey the closest thing Rome has had to a sole ruler since Sulla.

The atmosphere in Rome is tense. In the winding alleys of the city’s poorer quarters, violence erupts sporadically. A campaigner for the Caesarian faction is found dead next to a tavern, a Pompeian shopkeeper has his property burnt down, and gangs clash in the streets. A political slogan chanted or a drunken word of support for the wrong man can light the fuse, and blood is shed in an instant. Pompey is consul, but he is losing control of the streets, and the mob calls for Caesar.

Pompey brings charges against Caesar for his behaviour during the consulship in 59. This culminates in an order: Caesar must relinquish his command of the Gallic legions and return to Italy as a private and unarmed citizen to face trial for his crimes.

The threat of war hangs thick in the air, and when news comes of Vercingetorix’s surrender, the stage is set. Caesar begins his return home, six legions behind him, and Pompey and the optimates stand ready, awaiting their opponent’s first move.