Piso first came to notice in late 66 BC when he prosecuted
Gaius Manilius, a
plebeian tribune who was a supporter of Pompey. The prosecution was in retaliation for the
lex Manilia, which gave Pompey command of the Roman armies in the east during
the war against
Mithridates. Manilius was initially defended by
Cicero, but he dropped the case after the trial was violently disrupted by a paid mob. Piso pressed ahead with the trial, and Manilius fled the city ahead of a guilty verdict. "Carried away by his youthful enthusiasm", Piso leveled serious allegations at Manilius' powerful sponsor, Pompey, whom he disliked. Amused, Pompey asked Piso why he did not go further and prosecute him as well. Piso bitingly replied: Feeling threatened by populist politician and general Julius Caesar, the
optimates enlisted Pompey into their ranks in 53. In 50, the Senate, led by
Pompey, ordered Caesar to disband his army and return to Rome because his term as a governor had ended. Caesar thought he would be prosecuted if he entered Rome without the immunity enjoyed by a magistrate. On 10 January 49 he
crossed the Rubicon and ignited a
civil war. He marched rapidly on Rome and captured it. Pompey, the
optimates, and most of the Senate fled to Greece. Piso was sent to
Hispania Ulterior (in modern Spain). There he served as a
proquaestor under Pompey's
legates (legionary commanders)
Lucius Afranius and
Marcus Petreius. Taking advantage of Pompey's absence from the Italian mainland, Caesar made an astonishingly fast 27-day, west-bound forced march to Hispania and destroyed the Pompeian army in the
Battle of Ilerda. After the defeat of the Pompeian forces in Hispania, Piso escaped to
North Africa. There the
optimates raised an army which included 40,000 men (about eight
legions), a powerful cavalry force led by Caesar's former right-hand man, the talented
Titus Labienus, forces from local allied kings, and sixty
war elephants. This force was commanded by
Metellus Scipio, who placed Piso in command of the
Numidian cavalry. Caesar made an ill-planned and disorganised landing in Africa on 28 December 47. He had insufficient food and fodder, which forced him to break up his forces to forage. Piso's light cavalry effectively disrupted these efforts, notably at the
Battle of Ruspina when he harassed Caesar's defeated army as it retreated to its camp. The two armies continued to engage in small-scale skirmishes while Caesar waited for reinforcements. Then two of the
optimates' legions switched to Caesar's side. Emboldened, Caesar marched on
Thapsus and besieged the city at the beginning of February 46. The
optimates could not risk the loss of this position and were forced to accept battle. Scipio commanded "without skill or success", and Caesar won a
crushing victory which ended the war. Piso was forgiven in a general amnesty and seemed to come to terms with Caesar's victory. After Caesar's assassination in 44 BC, he joined with the tyrannicides,
Marcus Junius Brutus and
Gaius Cassius Longinus, during their civil war. They were defeated at the hard-fought
Battle of Philippi in 42, which involved 200,000 soldiers. Piso commanded troops during the campaign, but his precise role is not known. He was again pardoned and returned to Rome, where he refused to participate in the political arena which was under the control of Caesar's heir,
Octavian (later known as Augustus). ==Succession crisis of 23 BC==