After Gaius Julius Caesar conquered Gaul (roughly modern-day France and Belgium), his enemies in the Roman Senate declared him an enemy of the people - against the will of the people, for whom he was a hero, and against the will of the majority of the Senate, who simply wanted peace. Therefore, Caesar was forced to wage civil war against Gnaeus Pompeius, the head of the Roman oligarchy. Historians note the successful pacification of Gaul - during the civil war, which lasted for about four to five years, there were no major uprisings in Gaul. Although it would seem that the moment to recapture the lands for the Gauls was opportune. But I have never found a concrete explanation of how exactly Caesar managed to do this.

Finally I found the answer in Caesar's own "Commentaries on the Civil War". He simply took with him anyone who might still be willing to wage war to fight against Pompey:

"Caesar had sent six legions into Spain, five thousand infantry auxiliaries, three thousand cavalry, which he had had in all his previous wars, and an equal number from Gaul, which he had pacified, having summoned by name the most noble and bravest of all the cities, and added here men of the best kind from the Aquitanians and mountaineers who border the province of Gaul."

So, "Guys, why sit at home? Let's go beat up the oligarchs." The crowd flocked eagerly: turning from vanquished to victors was a good idea. Besides, Caesar was not only talented in military affairs, but also very generous when it came to dividing up the spoils.

P.S. This also explains why Caesar pardoned the future conspirators. In the case of the Gauls, in the case of the Germans across the Rhine, with whom he first fought and then recruited them into his cavalry, and in the case of most of his opponents, it really worked.