The coalition between Gaius Iulius Caesar, Cnaius Pompeius Magnus, and M. Licinius Crassus, formed in 60 BC. Caesar and Pompey were locked in a bitter rivalry at the time. Crassus, though not their equal politically, was admitted because of his great wealth.
The arrangement, though not secret, was a private one. It had no official standing under the laws of Rome. Its force derived purely from the power of the three men in it.
Under the terms of the triumvirate, Caesar was made consul in 59 BC, then given a governorship. The taxes he could collect there would replentish his drained fortune. Pompey received his fellow triumvirs' support in persuading the Senate to ratify his conquests in Asia Minor. As part of the bargain, Pompey married Caesar's daughter Julia.
The agreement was renewed in 56 BC, when the three men divided up the available military commands for the next five years. Pompey was put in charge of Spain, Crassus was given Syria, and Caesar's command in Gaul was extended.
Julia died in 54 BC, and Crassus in 53. At that point, the triumvirate dissolved, and the long-standing rivalry between Pompey and Caesar plunged Rome into bitter civil war.