Julius Caesar is still remembered for winning a civil war and helping bring about the end of the Roman Republic, leaving a line of emperors in its place, but it's quite possible that none of what Caesar did would've happened without the template for such actions being set 40 years earlier. At the time, when Caesar was in his teens, war was being waged both on the Italian peninsula and abroad, with domestic politics pitting the conservative, aristocratic optimates against the populist, reformist populares, and this tension ultimately escalated into an all-out war. One of the leading populares was Caesar's uncle, Gaius Marius, a military visionary who had restructured the legions and extended the privileges of land ownership and citizenship to legionaries on condition of successful completion of a fixed term of service. In the late 2nd century BCE, Marius had waged a successful campaign against several Germanic tribes, and after earning eternal fame in the Eternal City, Marius was appointed a consul several times. In 88 BCE, he entered into conflict with his erstwhile protégé, the optimate Sulla, over command of the army to be dispatched against Mithridates VI of Pontus, a long-time enemy of Rome and its Greek allies.

Ironically, Marius's reforms had made the legions fiercely loyal to their individual generals rather than the state, which allowed Sulla to march his army against Rome and force Marius into exile. With that, Rome's first civil war was officially underway, but Sulla's triumph proved short-lived. Just as Sulla departed for a campaign, Marius returned at the head of a scratch army of veterans and mercenaries, taking over the city and purging it of Sulla's optimate supporters, and though Marius died in 86 BCE, his party remained in power.