He had just had these significant military victories in Gaul and they said, alright Caesar, why don't you just leave your position, leave your army, and return to Rome? Well Caesar is sitting over here saying, well that doesn't really make a lot of sense. If I were to just return to Rome, they already are threatened by me. Who knows what they're going to do to me? So he decides to cross the Rubicon and enter Rome. And so the Roman senators, they say, okay, let's get Pompey, the famous general, who used to be part of Caesar's triumvirate but had switched sides, to engage with Caesar. But Pompey says, you know what, I'm not so sure if Caesar is beatable right now.