Why did Pompey side with the Roman Senate against Caesar?

Pompey the Great’s decision to side with the Roman Senate against Julius Caesar was the result of a gradual breakdown in their political alliance, fueled by mutual suspicion and shifting power dynamics.123

Several key factors drove this alignment:

• Collapse of Personal Bonds: The death of Julia—Caesar’s daughter and Pompey’s wife—in 54 BCE severed the strongest personal link between the two men.123
• Growing Distrust: Pompey became increasingly wary of Caesar’s rising power and military success in Gaul. Conversely, Pompey’s own legislative reforms in 52 BCE, such as laws against bribery and restrictions on provincial commands, were perceived by Caesar’s allies as direct attempts to undermine Caesar’s political security.43
• Political Necessity: As anarchy grew in Rome, the Senate turned to Pompey to restore order, appointing him sole consul in 52 BCE. This solidified his position within the senatorial establishment.42
• The Final Breach: When the Senate, led by Gaius Claudius Marcellus, formally requested that Caesar relinquish his command, Pompey accepted the Senate's invitation to raise an army to defend the state against his former ally.43