Growing Rivalry: Both Pompey and Caesar were ambitious and sought power and glory. Caesar's military successes in Gaul increased his fame and prestige, making him a potential rival to Pompey's influential position in Rome. The rivalry between the two men became more pronounced as Caesar gained support and loyalty from his legions.
The First Triumvirate: The First Triumvirate, an informal political alliance between Pompey, Crassus, and Caesar, had brought the three together against their common enemies. However, Crassus' death in 53 BCE created a vacuum in Roman politics, leading Pompey and Caesar to compete for dominance. Their interests collided, particularly regarding decisions relating to military matters and control over provinces.
Political Tensions: The Senate, dominated by conservative aristocratic factions, viewed Caesar's growing power with suspicion and sought to undermine him. Pompey initially remained neutral, trying to balance his alliance with Caesar and his standing with the Senate. However, the Senate played on Pompey's fears of Caesar's intentions, driving a wedge between them.
The Rubicon Crossing: In 49 BCE, Caesar defied the Senate by crossing the Rubicon River, essentially declaring war on Rome. He marched towards Rome with his loyal legions, forcing Pompey and the Senate to flee from the city. This act marked the beginning of the civil war between Caesar and Pompey's forces.
Personal Issues: On a personal level, Caesar and Pompey also had differences in personality and political ideology. Caesar was more charismatic and had a closer relationship with his soldiers, while Pompey was known for his military achievements but was somewhat aloof and hesitant in making decisions. These factors contributed to the eventual downfall of their alliance.
In summary, the alliance between Pompey and Caesar came apart due to their personal ambitions, growing rivalry, competing political interests, the changing political landscape in Rome, and internal tensions within their partnership. Ultimately, these factors led to the outbreak of civil war and the eventual victory of Caesar, solidifying his dominance over the Roman world.