After Crassus' departure from Rome at the end of 55 BCE and following his death in battle in 53 BCE, the First Triumvirate started to fracture more cleanly. With the death of Crassus, and that of Julia (Caesar's daughter and Pompey's wife) in 54 BCE, the balance of power between Pompey and Caesar collapsed and "a faceoff between two] may, therefore, have seemed inevitable". From 61 BCE, the main political fault-line in Rome was counterbalancing against the influence of Pompey, leading to his seeking allies outside the core senatorial aristocracy, i.e. Crassus and Caesar; but the rise of anarchic political violence from 55–52 BCE finally forced the Senate to ally with Pompey to restore order. The breakdown of order in 53 and 52 BCE was extremely disturbing: men like Publius Clodius Pulcher and Titus Annius Milo were "essentially independent agents" leading large violent street gangs in a highly volatile political environment. This led to Pompey's sole consulship in 52 BCE in which he took sole control of the city without convening an electoral assembly.

One of the reasons given as to why Caesar decided to go to war was that he would be prosecuted for legal irregularities during his consulship in 59 BCE and violations of various laws passed by Pompey in the late 50s, the consequence of which would be ignominious exile. Caesar's choice to fight the civil war was motivated mostly stumbling in efforts to attain a second consulship and triumph, in which failure to do so would have jeopardised his political future. Moreover, war in 49 BCE was advantageous for Caesar, who had continued military preparations while Pompey and the republicans had barely started preparing.

Even in ancient times, the causes of the war were puzzling and perplexing, with specific motives "nowhere to be found". Various pretexts existed, such as Caesar's claim that he was defending the rights of tribunes after they fled the city, which was "too obvious a sham".