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In the first book, Gates of Rome, the general is - though not given a role - a probably supporting general of the grand-general Sulla of the Roman conservatives, fighting the populares led by Cinna and Gaius Marius, the latter of whose sister's son (in reality wife's brother's son) was no other than Julius Caesar, then a teenager. Sulla arranged games and soon rose to consul, the second most powerful title in the Roman Republic, held by two men and the most powerful in peace. However, Sulla got in conflict with his co-consul Marius and was - after various intrigues and fightings - sent to Greece to fight Mithridates VI of Pontus. He had bribed senators to choose Marius' Primigenia legion to send, but with a few votes Marius won by selling his house and "outbribing" Sulla, he was sent away with Antonidus and their Second Aulelia legion instead. However, the military mastermind Sulla left soldiers inside Rome itself before marching. Fighting Mithridates (less than 20,000 soldiers clashed in battle) he won, and calmly returned to fight Marius in the battle of the Gates of Rome, where Marius was assassinated and the city with its empire conquered by Sulla, Antonidus and the conservatives. After banishing Caesar but sparing his life (claiming he didn't have to kill him, and then it would just be foolish to do so, indeed showing a bit of scruples in the otherwise so merciless ruler).
In the sequel Death of Kings, Antonidus plays a central role. He accompanies Sulla as the womanizing and alcoholic ruler is poisoned, an act of Tubruk, Caesar's and Brutus' earlier guardian when they were children (they are similar age in the novels, though Brutus were fifteen years younger and sometimes claimed to be Caesar's son - though unlikely - in reality). Antonidus partly takes over the power after the death of his master, but cannot prevent a bit of anarchy, though the dictatorship placing optimates in power was abolished. Upon Caesar's return to Rome (when he was about 25 years old) after fighting pirates in the Mediterranean, Antonidus blames him for acting without permission and murdering a Roman soldier in Libya. Antonidus and his supporter, the conservative Cato the younger is, however, pressured down by Pompey, as Antonidus and Cato had acted together in a plot where Cato hired an assassin to kill a loved one of all those believed by them to be involved in the Sulla assassination, as an act of revenge, but mostly to recognize and try to get out the real mastermind behind the assassination. Not able to guess at all the truth, the daughter of Pompey Magnus is killed, he himself then supports Caesar as Antonidus and Cato opposes his acts, where he once for all (ironically, as Caesar worshipped his uncle Marius and hated Sulla) finished Sulla's work, defeating Mithridates as he made rebellion again, Mithridates dying in the battle and handed over to Roman authorities for burial by cremation.
This is, in Iggulden's novel (though not historical accurate, supporting the thrilling plot), the first reason why Pompey and Caesar become friends. Pompey is also a close friend of Crassus, populare and the richest man in Rome. As the (implied to be) leper assassin paid by Cato is planning to murder Crassus' wife, Antonidus visits him and tells him to take "the other", aiming Caesar's loved and beautiful wife Cornelia, the daughter of Cinna, who earlier fought the conservatives with Marius. After Sulla's victory, the house of Marius was given to Antonidus, who now resides there, in Caesar's eyes illegally. Caesar smuggles weapons into the city and seizes the house, by arms but without blood. Antonidus is furious and has Caesar brought to court. Caesar has a magnificent speech, putting the people (who rather likes a populare than an optimate as Antonidus) on his side. However, Antonidus has the best arguments and hired lawyers, making the first two judges vote for him. The third and last rises, superior he puts in his veto for Caesar and thereby saves him from decapitation, and since Antonidus has lived in the house now declared to having belonged to Caesar for the last years Caesar demands to be paid as a landlord. Antonidus, lacking the resources paying for an extremely large and luxury house for almost ten years (having it bought rather than given, for ten denarii, about a dollar 2006) is sold as a slave, begging Cato for money but refused. He is then put on a farm in northern Italy, slaving for a rich lord in a garden. Then, suddenly he is saved when Spartacus with 80,000 slaves seizes parts of Italy, and Spartacus himself frees Antonidus in the yard. Not knowing who he had been (or not caring) he lets Antonidus have "an hour or two - alone - with the lord and his daughters", who were taken prisoners, presumably raping the daughters and maybe - but probably not - killing his lord. He then accompanied Spartacus when Crixus convinced him of that they should stay in Italy now when they were so successful, sacking and plundering, rather than escape to the free Gaul and Germania. Spartacus is convinced, and the slave army is led to Calabria where Crassus has them shut in with a wall to starve them. Spartacus hires pirates to transport them to Sicily, by the historians to create a slaveless independent state there, a sanctuary for his tens of thousands of men, but the pirates are defeated by Pompey's navy (while Caesar's wife and Tubruk are killed, Crassus later defeats the slave army and Spartacus is killed, Pompey tracks down the leper assassin and has him see his daughters die and then kills him in a personal vendetta, then has Cato executed, but in the last moment he slashes his own throat with a concealed knife). Antonidus however escapes during the naval battle on board a ship with the gold reserve of Spartacus, supposedly living rich in Algeria for the rest of his life.