FIRST Jeopardy 2.0! (11/25)

World rivers for 500, in the times of the Roman Empire, crossing this river with your army was considered an act of treason. Remember to PM me your answer in the form of a question within 50 minutes.

11/25

World rivers for 500, in the times of the Roman Empire, crossing this river with your army was considered an act of treason.

What is the Rubicon?