Plutarch of Chaeronea (46-c.122): influential Greek philosopher and author, well known for his biographies and his moral treatises. His biography is here; the following fragment is from his Life of Julius Caesar.
On 7 January 49 BCE, the Senate demanded Julius Caesar to hand over his ten well-trained legions to a new governor. Caesar heard the news in Ravenna, and knew that he had to make a choice between prosecution and rebellion; preferring the dignity of war over the humiliation of a process, Caesar chose to rebel, quoting his favorite poet Menander, "the die is cast" (alea iacta est).
On January 10 or 11, his army advanced to Rimini, where Caesar could control the passes across the Apennines: in doing so, he crossed the river Rubico, thereby invading Italy and provoking the Second Civil War. The bridge at modern Savignano probably marks the place where the ancient Via Aemilia crossed the Rubico.
The following story can be found in the Life of Caesar by the Greek author Plutarch of Chaeronea. The translation was made by Rex Warner.
 Yet the demands made by Caesar certainly looked fair enough. What he suggested was that he should lay down his arms and Pompey should do the same thing; they should then both, as ordinary private individuals, see what favor they could find from their fellow citizens. He argued that those who wanted him to be disarmed while Pompey's own forces were strengthened were simply confirming one man in the tyranny which they accused the other one of aiming at.
When [the tribune] Curio, on Caesar's behalf, put these proposals before the people, he was loudly applauded. Indeed some people actually loaded him with garlands of flowers as though he were some victorious athlete. [Marc] Antony, too, who was a tribune, produced in front of the people a letter which he had received from Caesar on these points and, in spite of the consuls' efforts to suppress it, read it aloud. In the Senate, however, Pompey's father-in-law, Scipio, proposed a motion that Caesar should be declared a public enemy if he had not laid down his arms before a certain date. And when the consuls put the question, first, whether Pompey should disband his troops, and then whether Caesar should, only a very few senators voted for the first proposal and nearly everyone voted for the second. But when Antony once more demanded that both should lay down their commands, the senate welcomed this proposal unanimously. Scipio however, violently protested against it and the consul Lentulus shouted out that in dealing with a robber what was required was arms, not votes. So for the time being the senate broke up and the senators put on mourning because of this failure to come to an agreement.
 Soon letters came from Caesar which were even more moderate in tone. He agreed to give up everything else, only asking for Cisalpine Gaul and Illyricum with two legions which he should retain till he stood for his second consulship. The orator Cicero [...] was working for a reconciliation and trying to make Pompey take up a less rigid attitude, and Pompey agreed to the proposals except that he still insisted that Caesar's soldiers should be taken from him. Cicero then approached Caesar's friends and tried to arrange a compromise by which they would agree to accept the provinces already mentioned and a force of only 6,000 soldiers. This was a figure which Pompey, on his side, was inclined to accept, but the consul Lentulus would not hear of it. He went out of his way to insult Antony and Curio and drove them out of the senate in disgrace. So of his own accord he gave Caesar the best possible excuse for taking action and supplied him with excellent material for propaganda among his troops. For Caesar could now show his soldiers these distinguished men of high office in the state who had fled from Rome in hired carts and dressed as slaves, as they had had to do in their fear when they slipped out of the city.
 Caesar had with him at the time no more than 300 cavalry and 5,000 legionary soldiers. The rest of his army had been left on the other side of the Alps and was to be brought up to him by officers who had been sent back to do so. He saw, however, that the very beginning and the first stages of his enterprise did not require the use of large forces for the time being. Better results could be obtained by surprise, daring, and taking the quickest advantage of the moment; it would be easier, he thought, to strike panic into his enemies by acting in a way which they never expected than it would be to force them back after having first made all the preparations for a regular invasion. So he ordered his centurions and other officers to take just their swords, leaving their other arms behind, and to occupy the large Gallic city of Rimini; they were to avoid all disturbance and bloodshed as far as they possibly could. He put Hortensius in command of this force and himself spent the day in public, watching gladiators at their exercises. In the late afternoon he had a bath, dressed, and went into the banqueting hall where he spoke for a little time with the guests who had been invited to dinner. When it was beginning to get dark he rose from the table and, after addressing a few polite words to the majority of his guests, whom he begged to remain there until he came back, he went away. He had already given instructions to a few of his friends to follow him, not all on the same route, but some on one way and some on another.
He himself got into one of the hired carriages and, setting out at first on a different road, finally turned and took the road to Rimini. When he came to the river (it is called the Rubico) which forms the frontier between Cisalpine Gaul and the rest of Italy he became full of thought; for now he was drawing nearer and nearer to the dreadful step, and his mind wavered as he considered what a tremendous venture it was upon which he was engaged. He began to go more slowly and then ordered a halt. For a long time he weighed matters up silently in own mind, irresolute between the two alternatives. In these moments his purpose was constantly changing. For some time too he discussed his perplexities with his friends who were there, among whom was [the future historian] Asinius Pollio. He thought of the sufferings which his crossing the river would bring upon mankind and he imagined the fame of the story of it which they would leave to posterity. Finally, in a sort of passion, as though he were casting calculation aside and abandoning himself to whatever lay in store for him, making use too of the expression which is frequently used by those who are on the point of committing themselves to desperate and unpredictable chances, 'Let the die be cast,' he said, and with these words hurried to cross the river.
From now on he marched at full speed and before dawn made his way into and occupied Rimini. It is said too that on the night before he crossed the river he had an unnatural dream. He dreamed that he was committing incest with his own mother.
 Rimini was captured and the broad gates of war were opened on every land and sea alike. The boundaries of the province were down and so was all law and order in the state. Men and women had, on other occasions in the past, fled from one part of Italy to another in terror; but now the impression was rather one of whole cities on the move in a panic-stricken course from one site to the next. Rome was, as it were, inundated as people came in from all the surrounding towns, escaping from their homes. The authority of magistrates and the eloquence of orators were ineffective to exert control, and in this great and stormy tempest the city nearly allowed itself to go under. On every side violently opposed feelings were expressed in violent action. Those who were pleased with what happened did not keep their feelings to themselves; they were constantly meeting, as was inevitable in a large city, others who viewed the situation with fear or anger, and their own easy confidence with regard to the future naturally led to quarrels.
Pompey's own state of mind was already sufficiently disturbed and it was made all the more confused by what he had to listen to from other people. Some attacked him for having armed Caesar against himself and the state, while others blamed him for having allowed Lentulus to insult Caesar just at the time when Caesar was prepared to give way and to accept a reasonable settlement. Favonius told him that now was the time for him to stamp on the ground - a remark prompted by the fact that Pompey had previously made a boastful kind of speech to senate in the course of which he had said that there was no need for them to waste their time bothering about preparations for the war, since, when it came, he had only to stamp with his foot upon earth in order to fill the whole of Italy with armies.
Even so, Pompey at this time had more troops available to him than Caesar had. But no one would allow him to use his own judgment. Inaccurate and panic-stricken reports kept on coming in to effect that Caesar was already close at hand and sweeping everything before him. Under the influence of these reports Pompey gave and allowed himself to be carried along in the general stream and issued an edict declaring that the city was in a state of anarchy, and abandoned Rome. His orders were that the senate should follow and that no one should remain behind except those who preferred tyranny to freedom and to their own country.