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 river, the boundary of Italy, and ignited the Great Roman 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, a type of military auditor, 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.