Don’t cross the Rubicon is what Rome told Spartacus.

Spartacus had escaped, formed his own army and had kicked the shit out of every General Rome had sent against him. He fought his way north and was headed toward Germania (or some such shithole place). Then, he turned around and headed back into what is now Italy.

Rome told him to keep going north and if he crossed the Rubicon (southbound) they’d end his dumb ass for him.

Of course Spartacus crossed the Rubicon and Rome chased his ass down and then crucified not only him but all his men. They not only crucified them, but they left them up on the crosses for the vultures to feast on and for all the people to see - this is what happens to you if you disobey Rome.