Cassius was elected as a Tribune of the Plebs in 49 BC. He opposed Caesar, and eventually he commanded a fleet against him during Caesar's Civil War: after Caesar defeated Pompey in the Battle of Pharsalus, Caesar overtook Cassius and forced him to surrender. After Caesar's death, Cassius fled to the East, where he amassed an army of twelve legions. He was supported and made Governor by the Senate. Later he and Brutus marched west against the allies of the Second Triumvirate.