Caesar's conquest of Gaul led to the subjugation of the Celtic people, who started to rebel after hearing rumors of political troubles back in Rome. The rebellion gained steam as tribes joined in and yielded massive casualties, except for the loyal ones to Rome.
The Gallic tribes joined forces to plan an attack against Roman rule, with the druids officiating the event. These tribes made promises and offered rewards of every kind to induce volunteers to strike the first blow and risk their lives to restore the liberty of Gaul.
Caesar saw the Gauls as ultimately assimilable into the Roman empire, but some historians argue that he may have misjudged the severity of the conflict. Nonetheless, he continued to make the Celtic side appear admirable in order to set the stage for possible incorporation into Rome in the future.
The rebellion against Rome by the Gallic tribes started with a massacre of Romans in a hillfort town which signaled to other tribes to join the fight. As the rebellion gained steam, even previously friendly tribes of Rome sided with the Gallic tribes.