In 49BC, Julius Caesar was a General who had just annexed Gaul (modern day France) into the Roman Empire. Caesar made himself the Governor of Gaul, where he was popular with the people and his army of 10 legions. The Roman Senate, fearing Caesar’s popularity, issued an ultimatum that Caesar was not to cross the border between Gaul and Rome marked by the River Rubicon, else he would be named a traitor and spark a civil war between Caesar and the Senate. Aside from Caesar’s 10 legions, the Roman military had 20 other legions who were controlled by various other Generals.