If he returned to Rome without an army, Caesar would have been prosecuted for corrupt consuling and also probably exceeding his authority as governor. So instead, he returned with the 13th Legion. He crossed the Rubicon River, famously saying, the die is cast, or possibly, let the die be cast. Sorry, Thought Bubble, sources disagree. Basically, Caesar was invading his own hometown. Pompey was in charge of Rome's army, but like a boss, fled the city. And by 48 BCE, Caesar was in total command of all of Rome's holdings, having been named both dictator and consul.