Piso first came to notice in late 66 BC when he prosecuted Gaius Manilius (also known as Gaius Manilius Crispus), a plebeian tribune who was a supporter of Pompey. The prosecution was political; Manilius had passed laws which the "optimates" disapproved of. Specifically 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: