Victory abroad brought crisis at home. In its last century the Republic's institutions, built for a small city-state, buckled under the strains of empire, and Roman politics slid from argument into bloodshed.
The Gracchi
The wars had ruined many small farmers: away on campaign for years while wealthy landowners bought up estates worked by slaves. In 133 BC the tribune Tiberius Gracchus proposed redistributing public land to the poor. His methods alarmed the Senate, and he was clubbed to death by a mob of senators: the first time in centuries that Roman political conflict turned openly murderous. His brother Gaius revived the cause a decade later and met the same end. Reform had become a killing matter.
Marius and the army
The general Gaius Marius then changed the state's foundations almost by accident. Needing soldiers, he opened the legions to the landless poor, who had nothing to return to and everything to gain from a successful commander. Armies began to look to their generals (for pay, for plunder, for land in retirement) rather than to the Senate. The legions were becoming private followings.
Sulla marches on Rome
The consequences arrived with Sulla. In 88 BC, in a dispute over a military command, he did the unthinkable: he led his army against Rome itself, something no Roman general had ever dared. He later returned, won a civil war, and had himself made dictator (not for six months in the old emergency style, but indefinitely) and posted lists of enemies, the proscriptions, marking thousands for death and confiscation.
The lesson learned
Sulla eventually resigned and tried to restore the Senate's power, but the precedent could not be undone. He had shown that a general with a loyal army could simply take the state by force, and that political enemies could be murdered wholesale under colour of law. The ambitious men who came after him (Pompey, Crassus, Caesar) had watched, and learned.