Although Marty is annoyed at Bifficus' attitude, Jules and Verne warn that they could indirectly ruin Hill Valley as they could return to find no United States, finding that instead it would be under a still-existent Roman Empire. Ergo, there would be no way of knowing if Hill Valley would have been discovered by the Romans, or if so, whether the Hill Valley citizens would be oppressed as many people were by the Ceasars.