It so happened that many historical events not only settle in writings, annals and chronicles, but are also firmly fixed in live speech, and even people who have not heard of the real background of phraseological units can use them. So it happened with the famous passage of Caesar across the legendary river. The commander decided to cross the Rubicon, the idiom remained in the speech of his descendants.
This river is now called Fiumicino, flows into the Adriatic and flows between two Italian cities: Rimini and Cesena. Its name was born from "rubeus" (that is, "red" in Latin, because its waters flow on clay soils). Now it is a small river, almost drying up, because its waters have been used to irrigate fields for many centuries. But in the time of Caesar, it was along the reddish rivulet that the then border between Italy itself and one of the Roman lands, Cisalpine Gaul, passed. Gaius Julius, then proconsul, commanded the 13th Double Legion and was obliged to stop by the river: after all, the proconsul could only command soldiers in the provinces and could not lead legions in the lands of Italy proper. This iswould be a direct violation of the law and the powers of the Senate, a state crime and therefore punishable by death. But, alas, there was no other choice.
Then Caesar fought for power with the Senate of Rome, having received the province of Gaul in control. The famous commander did not immediately decide on hostilities, he could go to various agreements, so long as there was no bloodshed, and even dragged out the negotiations with all his might, postponing the start of the actual hostilities. However, his efforts did not lead to success, too many wanted war. His opponent was Pompey, who had a huge Roman army.
Caesar's position was not particularly rosy: the bulk of his army was behind the Alps. Fast moves and decisive options were needed, there was no time to wait for reinforcements. Therefore, in January 49 BC, Gaius Julius ordered his commanders to cross the Rubicon and occupy the town of Armin, which was located south of the mouth of the river. This demarche called him not just to cross the Rubicon, the significance of this step was enormous.
The brilliant military leader was able to defeat the forces of the Senate and become the sovereign and sole ruler of the Eternal City, because the opponents panicked and fled as soon as they heard about Caesar's demarche. For himself, this transition was also a fateful event.
If you believe the story of the historian Suetonius, having decided to cross the Rubicon, the commander even said: "The die is cast." After the victory Gaius Julius Caesar was able towin not only the love of the people, but also created a powerful state that lasted another fifty years.
Since then, the expression "to cross the Rubicon" has become a catchphrase, which means making a decisive act, making a fateful decision. That is, this is a kind of significant step, forever dividing events into “before” and “after”, radically changing the state of affairs. There is no turning back after such a decision. The expression is quite old, common in many languages of the world.