As he thinks in Nestor about how the course of history might have been different if Pyrrhus had not died in Argos, Stephen jumps to the more famous example of Julius Caesar's assassination in 44 BC by 60 knife-wielding Roman senators. Caesar had recently been declared Dictator of the Roman Republic, and the conspirators feared that he aspired to become King, but the years after his death saw the Republic replaced by monarchy anyway. Stephen may well wonder what might have happened if Caesar had "not been knifed to death."
In Eumaeus, Stephen asks Bloom to "oblige me by taking away that knife. I can't look at the point of it. It reminds me of Roman history." Apparently he is still thinking of Pyrrhus and Caesar.