This action made Sertorius highly renowned throughout all Spain, and as soon as be returned to Rome he was appointed quaestor of Cisalpine Gaul, at a very seasonable moment for his country, the Marsian war being on the point of breaking out. Sertorius was ordered to raise soldiers and provide arms, which he performed with a diligence and alacrity, so contrasting with the feebleness and slothfulness of other officers of his age, that he got the repute of a man whose life would be one of action. Nor did he relinquish the part of a soldier, now that he had arrived at the dignity of a commander, but performed wonders with his own hands, and never sparing himself, but exposing his body freely in all conflicts, he lost one of his eyes. This he always esteemed an honour to him; observing that others do not continually carry about with them the marks and testimonies of their valour, but must often lay aside their chains of gold, their spears and crowns; whereas his ensigns of honour, and the manifestations of his courage, always remained with him, and those who beheld his misfortune must at the same time recognize his merits. The people also paid him the respect he deserved, and when he came into the theatre, received him with plaudits and joyful acclamations, an honour rarely bestowed even on persons of advanced standing and established reputation. Yet, notwithstanding this popularity, when he stood to be tribune of the people, he was disappointed, and lost the place, being opposed by the party of Sylla, which seems to have been the principal cause of his subsequent enmity to Sylla.
After that Marius was overcome by Sylla and fled into Africa, and Sylla had left Italy to go to the wars against Mithridates, and of the two consuls Octavius and Cinna, Octavius remained steadfast to the policy of Sylla, but Cinna, desirous of a new revolution, attempted to recall the lost interest of Marius, Sertorius joined Cinna's party, more particularly as he saw that Octavius was not very capable, and was also suspicious of any one that was a friend to Marius. When a great battle was fought between the two consuls in the forum, Octavius overcame, and Cinna and Sertorius, having lost not less than ten thousand men, left the city, and gaining over most part of the troops who were dispersed about and remained still in many parts of Italy, they in a short time mustered up a force against Octavius sufficient to give him battle again, and Marius, also, now coming by sea out of Africa, proffered himself to serve under Cinna, as a private soldier under his consul and commander.
Most were for the immediate reception of Marius, but Sertorius openly declared against it, whether he thought that Cinna would not now pay as much attention to himself, when a man of higher military repute was present, or feared that the violence of Marius would bring all things to confusion, by his boundless wrath and vengeance after victory. He insisted upon it with Cinna that they were already victorious, that there remained little to be done, and that if they admitted Marius, he would deprive them of the glory and advantage of the war, as there was no man less easy to deal with, or less to be trusted in, as a partner in power. Cinna answered, that Sertorius rightly judged the affair, but that he himself was at a loss, and ashamed, and knew not how to reject him, after he had sent for him to share in his fortunes. To which Sertorius immediately replied, that he had thought that Marius came into Italy of his own accord, and therefore had deliberated as to what might be most expedient, but that Cinna ought not so much as to have questioned whether he should accept him whom he had already invited, but should have honourably received and employed him, for his word once passed left no room for debate.
Thus Marius being sent for by Cinna, and their forces being divided into three parts, under Cinna, Marius, and Sertorius, the war was brought to a successful conclusion; but those about Cinna and Marius committing all manner of insolence and cruelty, made the Romans think the evils of war a golden time in comparison. On the contrary, it is reported of Sertorius that he never slew any man in his anger to satisfy his own private revenge, nor ever insulted over any one whom he had overcome, but was much offended with Marius, and often privately entreated Cinna to use his power more moderately. And in the end, when the slaves whom Marius had freed at his landing to increase his army, being made not only his fellow-soldiers in the war, but also now his guard in his usurpation, enriched and powerful by his favour, either by the command or permission of Marius, or by their own lawless violence, committed all sorts of crimes, killed their masters, ravished their masters' wives and abused their children, their conduct appeared so intolerable to Sertorius that he slew the whole body of them, four thousand in number, commanding his soldiers to shoot them down with their javelins, as they lay encamped together.