10 January 49 B.C.
Ah, what a fine day to commit treason against the Roman Republic! With a fun battle cry, "alea jacta est"¹ Caesar crossed the small stream called the Rubicon that separated Italy from the northern province of Cisalpine Gaul, thereby violating the Roman law that said a general could not bring his army out of the province to which he was assigned. He had been in Gaul for nine years, killing a lot of Gauls, and he had become very famous in Rome. The Senate, jealous of his power, demanded that he give up his legions, but he knew they would try to ruin him if he did that. So he took his armies into Rome, and initiated a three-year civil war that destroyed the Republic (which was falling apart anyway) and set the stage for the Empire. Caesar did okay for himself, too, becoming virtual dictator. Until, you know, he was assassinated. This is such a decisive moment in history that it has entered the regular lexicon when anyone does something bold and irreversible. Use it in conversation!
You: "You know, [insert your boss's name here], you're a jackass."
Your boss: "Boy, you really crossed the Rubicon with that, didn't you?"
¹ "The die is cast." Come on, people, Latin is awesome!