Basically he broke the long-time rule about bringing troops into Rome other than the Praetorian Guard. This was such a break in tradition that it left no room for doubt that he was willing to do WHATEVER it took to overthrow the current rule by the Senate. There was no going back from it - Ceasar had put it all on the table. Think Hilary or Guiliano publicly procaliming that they were going to literally tear up the US Constitution if elected President....Ceasar crossing the Rubicon was similarly shocking. challanging and indicitice of how he felt about things.

The crossing of a small stream in northern Italy became one of ancient history's most pivotal events. From it sprang the Roman Empire and the genesis of modern European culture. Born with unbridled political ambition and unsurpassed oratory skills, Julius Caesar manipulated his way to the Roman Legionary position of consul of Rome in 59 BC. After his year of service he was named governor of Gaul where he amassed a personal fortune and exhibited his outstanding military skill in subduing the native Celtic and Germanic tribes. Caesar's popularity with the people soared, presenting a threat to the power of the Senate and to Pompey, who held power in Rome. Accordingly, the Senate called upon Caesar to resign his command and disband his army or risk being declared an "Enemy of the State". Pompey was entrusted with enforcing this edict - the foundation for civil war was laid. It was January 49 BC, Caesar was staying in the northern Italian city of Ravenna and he had a decision to make. Either he acquiesced to the Senate's command or he moved southward to confront Pompey and plunge the Roman Republic into a bloody civil war. An ancient Roman law forbade any general from crossing the Rubicon River and entering Italy proper with a standing army. To do so was treason. This tiny stream would reveal Caesar's intentions and mark the point of no return. The Die is Cast Suetonius was a Roman historian and biographer. He served briefly as secretary to Emperor Hadrian (some say he lost his position because he became too close to the emperor's wife.) His position gave him access to privileged imperial documents, correspondence and diaries upon which he based his accounts. For this reason, his descriptions are considered credible. We join Suetonius's narrative as Caesar receives the news that his allies in the Senate have been forced to leave Rome: "When the news came [to Ravenna, where Caesar was staying] that the interposition of the tribunes in his favor had been utterly rejected, and that they themselves had fled Rome, he immediately sent forward some cohorts, yet secretly, to prevent any suspicion of his plan; and to keep up appearances, he attended the public games and examined the model of a fencing school which he proposed building, then - as usual - sat down to table with a large company of friends. However, after sunset some mules from a near-by mill were put in his carriage, and he set forward on his journey as privately as Julius Caesar possible, and with an exceedingly scanty retinue. The lights went out. He lost his way and wandered about a long time - till at last, by help of a guide, whom he discovered towards daybreak, he proceeded on foot through some narrow paths, and again reached the road. Coming up with his troops on the banks of the Rubicon, which was the frontier of his province, he halted for a while, and revolving in his mind the importance of the step he meditated, he turned to those about him, saying: 'Still we can retreat! But once let us pass this little bridge, - and nothing is left but to fight it out with arms!' Even as he hesitated this incident occurred. A man of strikingly noble mien and graceful aspect appeared close at hand, and played upon a pipe. To hear him not merely some shepherds, but soldiers too came flocking from their posts, and amongst them some trumpeters. He snatched a trumpet from one of them and ran to the river with it; then sounding the "Advance!" with a piercing blast he crossed to the other side. At this Caesar cried out, 'Let us go where the omens of the Gods and the crimes of our enemies summon us! THE DIE IS NOW CAST!' Accordingly he marched his army over the river; [then] he showed them the tribunes of the Plebs, who on being driven from Rome had come to meet him, and in the presence of that assembly, called on the troops to pledge him their fidelity; tears springing to his eyes [as he spoke] and his garments rent from his bosom." References: Duruy, Victor, History of Rome vol. V (1883); Suetonius "Life of Julius Caesar" in Davis, William Stearns, Readings in Ancient History (1912).