In 50 BCE, the Senate dominated by Pompey the Great ordered Julius Caesar to return to Rome, dissolve his army, and forbade him to run for the second consulate. These activities were clearly dictated by the desire to diminish the political and military role of Caesar.
With the death of Crassus in 53 BCE. The triumvirate has ceased to function. Caesar was afraid that the Senate, dominated by optimism forces, would start proceedings against him, and Pompey would accuse him of betrayal and insubordination.
On January 7, 49 BCE, the Senate ordered Julius Caesar to hand over his army to the command of a new governor in Gaul. Caesar found out about this information in Ravenna.
Rubikon was recognized by the Senate as a border dividing the province of Pre-Galalia (Gallia Cisalpina) and Italy, which was under Rome and allies (socii). This border was in the northeast; the other border was the Arno River, much wider in the northwest, which flows from the Apennines to the Tyrrhenian Sea.
According to Roman law of the republic, the governors of the province were officials with imperium (the right to command an army) within their provinces. So they couldn’t operate their forces outside their sphere of influence. Roman law only allowed consuls and praetors to have an imperium in Italy. If the general did not have special powers to be able to enter the Italian lands, he first had to dissolve the army. A person who did not respect this law and entered Italy at the head of the army automatically lost the empire and command of the army and was sentenced to death. Moreover, soldiers who maintained their obedience to the commander without official permission were also subject to the death penalty.
Caesar had to choose between submitting to the will of the Senate and be publicly humiliated by a series of trials, or to rebel and start a rebellion. He decided to finally fight political opponents. He then said the famous words: Alea iacta est! (“The dice is cast”).
On January 10 or 11 of 49 BCE, Caesar, at the head of the army, reached Rimini, where, in order to secure the passage through the Apennines, he crossed the Rubicon, thus starting the civil war. The place where Caesar probably crossed the river is the current bridge in the city of Savignano in Italy. The ancient Via Aemilia was probably walking this way.
Rubicon crossed at the head of one legion – XIII Gemina. Suetonius mentions that Caesar was indecisive in making decisions and ultimately supernatural. The night after crossing the river, Caesar was supposed to feast with Sallustius, Hirtius, Oppius, Lucius Balbus and Sulpicus Rufus.
The speed and determination of Caesar forced Pompey, consuls C. Claudius Marcellus and L. Cornelius Lentulus Crus and part of the senate to leave Rome. Caesar’s decision turned out to be a great step for the Republic to fall.