On this day in history, 49 BC, Julius Caesar crossed the Rubicon with a legion of his soldiers, which was against Roman law.  Specifically, Governors of Roman provinces (promagistrates) were not allowed to bring any part of their army within Italy itself and, if they tried, they automatically forfeited their right to rule, even in their own province.  The only ones who were allowed to command soldiers in Italy were consuls or preators.  This act of leading his troops into Italy would have meant Caesar’s execution and the execution of any soldier who followed him, had he failed in his conquest. Caesar was initially heading to Rome to stand trial for various charges, by order of the Senate.  According to the historian Suetonius, Caesar wasn’t at first sure whether he’d bring his soldiers with him or come quietly, but he ultimately made the decision to march on Rome.

Shortly after the news hit Rome that Caesar was coming with an army, many of the Senators, along with the consuls G. Claudius Marcellus and L. Cornelius Lentulus Crus, and Gnaeus Pompeius Magnus, a.k.a. Pompey (Caesar’s chief rival for power who was supporting the Senate), fled Rome.  Somewhat humorously, they were under the impression that Caesar was bringing nearly his whole army to Rome.  Instead, he was just bringing one legion, which was largely outnumbered by the forces Pompey and his allies had at their disposal.  Never-the-less, they fled and after a four year struggle, Caesar was victorious and Pompey fled to Egypt where he was assassinated. Caesar then became Dictator Perpetuus of Rome.  This appointment and changes within the government that happened in the aftermath ultimately led to the end of the Roman Republic and the beginning of the Roman Empire.

Interestingly, despite the Rubicon once signifying the boundary between Cisalpine Gaul and Italy proper, the exact location of the river was lost to history until quite recently.   The river’s location was initially lost primarily because it was a very small river, of no major size or importance, other than as a convenient border landmark. Thus, when Augustus merged the northern province of Cisalpine Gaul into Italy proper, it ceased to be a border and which river it was exactly gradually faded from history.

Thanks to occasional flooding of the region until around the 14th or 15th centuries, the course of the river also frequently changed with very little of it thought to still follow the original course, excepting the upper regions.  In the 14th and 15th centuries, various mechanisms were put in place to prevent flooding and to regulate somewhat the paths of many rivers in that region to accommodate agricultural endeavors.   This flooding and eventual regulation of the rivers’ paths further made it difficult to decipher which river was actually the Rubicon.

Various rivers were proposed as candidates, but the correct theory wasn’t proposed until 1933, namely what now is called the Fiumicino with the crossing likely being somewhere around the present day industrial town of Savignano sul Rubicone (which incidentally was called Savignano di Romagna, before 1991).  This theory wasn’t proven until about 58 years later in 1991 when scholars, using various historical texts, managed to triangulate the exact distance from Rome to the Rubicon at 199 miles (320 km).  Following Roman roads of the day and other evidence, they then were able to deduce where exactly the original Rubicon had been and which river today was once the Rubicon (the Fiumicino river today is about 1 mile away from where the Rubicon used to flow around that crossing site).