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Like his brother, Gaius started his life free from paternal guidance, in the midst of scandals, parties and gambling.
During the civil war, Gaius was a legate of Julius Caesar (49 BC) and was entrusted with the defense of Illyria against Pompeians and with the campaign to regain cities already taken by rebels and the Pompeians. Gaius was entrusted with the newly recruited 24th legion and half of the new 28th legion. On the way to Illyria, Antonius was intercepted by a Pompeian fleet. Led by centurion Titus Pullo, the men refused to fight, and turned traitor. They were then reported to be fighting at the Battle of Dyrrachium.
With all the members of the Antonius family, he was then promoted to high offices of the cursus honorum. In 44 BC, Gaius was urban praetor, while his brothers Mark Antony and Lucius Antonius were consul and tribune respectively.
Caesar's assassination and afterwards
After the assassination of Caesar, Gaius (as a Caesarean) was appointed governor to the Roman province of Macedonia. Marcus Junius Brutus and the other assassins, however, chose Macedonia as refuge from Octavian and - following opposition from Gaius - dispossessed him of his governorship. Brutus at first seemed to treat him generously, but, on finding that he was attempting to persuade his troops to mutiny, ultimately ordered his death.