Escaped gladiators led by Spartacus raise an army of slaves and beat several Roman armies before Crassus crushes them. Plutarch says the young Cato served in the war as a volunteer.
When the servile war was in progress, which was called the war of Spartacus, Gellius was commander, while Cato took part in his campaign as a volunteer, for the sake of his brother; for his brother Caepio was a military tribune.
Crassus and Pompey used their victories to win the consulship of 70 BCE, and their actions as consuls helped transform the Roman Republic into the Roman Empire.
Based on Wikipedia: Third Servile War
Pompey and Crassus exploited their successes to further their political careers, using their public acclaim and the implied threat of their legions to sway the consular elections of 70 BCE in their favor.
Their actions as consuls greatly furthered the subversion of Roman political institutions and contributed to the transformation of the Roman Republic into the Roman Empire.