The First Triumvirate was an informal political alliance among three prominent politicians in the late Roman Republic: Gaius Julius Caesar, Gnaeus Pompeius Magnus and Marcus Licinius Crassus. The constitution of the Roman republic had many veto points. In order to bypass constitutional obstacles and force through the political goals of the three men, they forged in secret an alliance where they promised to use their respective influence to support each other. The "triumvirate" was not a formal magistracy, nor did it achieve a lasting domination over state affairs.

It was formed between the three men due to their mutual need to overcome opposition in the senate against their proposals in the previous years. Initially secret, it emerged publicly during Caesar's first consulship in 59 BC to push through legislation for the three allies. Caesar secured passage of an agrarian law which helped resettle Pompey's veterans, a law ratifying Pompey's settlements after the Third Mithridatic War, and legislation on provincial administration and tax collection. Caesar also was placed in a long-term governorship in Gaul. The early success of the alliance, however, triggered substantial political backlash. Political alliances at Rome reorganised to counterbalance the three men in the coming years.

By 55 BC, the alliance was fraying. The three men, however, came together in mutual interest to renew their pact. By force and with political disruption aided by their allies, they delayed consular elections into 55 BC and intimidated the comitia into electing Pompey and Crassus again as consuls. Caesar's command in Gaul was then renewed for another five years; plum provincial commands placed Pompey in Spain and Crassus in Syria. Amid even stronger backlash at Rome against the use of naked force and chaos to achieve political ends, Crassus was killed during his ill-fated invasion of Parthia in 53 BC.

Caesar and Pompey, the two remaining allies, maintained friendly relations for a few years. They remained allies even after Pompey's assumption of a sole consulship in 52 BC and the death of Julia (Caesar's daughter and Pompey's wife). Pompey, however, moved to form alliances to counterbalance Caesar's influence after Crassus' death. These drew him slowly into a policy of confrontation with Caesar. Deteriorating trust through 50 BC, along with the influence of Catonian anti-Caesarian hardliners on Pompey, eventually pushed Caesar into open rebellion in January 49 BC.