After Caesar spent 51 BC and the better part of 50 BC touring his newly conquered province of Gaul, political chaos was developing back in Rome. The optimates despised Caesar and his conquests (viewing much of his campaigning as unwarranted and illegal) and looked for every opportunity to strip him of his command. These conquests not only brought in a great number of slaves, but brought so much monetary wealth into Rome, that the value of gold was actually reduced by as much as 1/4 or even 1/3 of its value before the wars. Though the common people noticed little effect, and seemingly loved Caesar, the optimates had a great deal to lose from this devaluation of their wealth. That was only one small piece of the puzzle, however. Caesar's original Consulship in 59 BC was one in which he not only thwarted optimate interests but pushed forward a populares agenda that not only made life miserable for the conservatives but generally rejected the law and political custom. Such actions were destabilizing and dangerous for the health of the Republican system.
They wanted to prosecute Caesar for a variety of reasons, including conducting an illegal war into Germania that the Senate never authorized. In fact, many argued that the protection of Cisalpine Gaul and Narbonensis didn't require the war that Caesar conducted in the larger part of Gaul in the first place. Prosecuting Caesar, whether the goal was death, exile or just a symbolic limitation of his power, would prevent his re-establishment of the populares agenda that he so masterfully manipulated previously. The years 50 and 49 BC were pivotal because during this time frame, Caesar's 'imperium' or safety from prosecution was set to expire. Caesar badly desired the ability to run for the Consulship in abstentia, thereby allowing him the safe transfer of protection from his Proconsular Imperium, granted by his command in Gaul, to that of the actual Consulship once again.
By this time, however, Pompey, likely the only man able to smooth things over, had clearly sided with the optimates. His jealously over Caesar's success and his ultimate goal of acceptance and power within the Senate took him ever further from the alliance with Caesar. The Lex Pompeia De Magistratibus that was passed while Pompey was Consul without colleague forced a candidate to be present in Rome to run for office, and of course, one couldn't legally bring their legions to Rome for protection. Marcus Claudius Marcellus, Consul for 51 BC, tried to have Caesar recalled from his command prior to its actual legal expiration, and in 50 BC, his cousin Gaius Claudius Marcellus attempted to the same. Caesar's only hope in Rome from a legal standpoint was the intervention of the Tribunes of the Plebes, who managed to veto any attempts to bring Caesar to his knees.
The situation continued in a virtual stalemate, with neither side willing to budge on their legal standpoints. More optimate officials were elected in 50 BC to take office for 49, but Caesar still managed to hold ground. His legate Marcus Antonius was elected as Tribune for the same year, and a former opponent, Gaius Scribonius Curio was also elected but paid handsomely to side with Caesar. 49 BC was shaping up to be yet another stalemate politically for Caesar, but he was quite simply running out of time. Already, in the autumn of 50 BC, in an effort to weaken Caesar, the Senate asked both he and Pompey to give up one legion each (Pompey had his armies in Spain) to secure the east against Parthia. In a wise move, the Senate boldly stripped Caesar of 2 legions though, 1 of his own, and one that Pompey had lent Caesar several years earlier during the Gallic revolt. Caesar was left with 8 legions and the legions that he had given up were never sent to Parthia. They stayed in Italy and were given to Pompey in a shrewd move that strengthened Pompey while weakening Caesar.
By late 50 BC, various attempts were made by the Consul Marcellus to stifle Caesar's tribunes and allow some measures of anti-Caesarean policy to go through. Curio however turned the tables and forced a unique vote to the senate floor. On December 1, 50 BC, Curio proposed a motion that would force both Pompey and Caesar to simultaneously lay down their commands, and the motion was passed 370 votes to 22. Though the Senate vote indicated that civil war was trying to be avoided, the hatred and/or political fear of Caesar simply couldn't compromised. The following day, before the measure could even be brought to Caesar, Consuls Gaius Claudius Marcellus and Lucius Cornelius Lentulus Crus ignored the Senate vote. In what was certainly an illegal action, in an era filled with political illegalities, the consuls ordered Pompey to take up command of the local legions and to raise more in defense of Rome against Caesar. The people of Rome (via the tribunes) were ignored and even the Senate in this case, though it's assured that the optimates played a major part. Certainly fearing the worst, Cicero tried to counteract this measure and avert what appeared to be a growing danger of civil war. He met with and attempted to influence Pompey into working for a compromise, but Pompey refused, and meanwhile, Caesar waited in Gaul.
Though Caesar held Rome in a precarious position by the strength of his army and the continuation of his political agenda, the Senate must hold some blame for pushing a known radical into a no-win situation. Caesar's options in these later developments were either to surrender willingly and face certain prosecution (though the outcome of such prosecution could certainly have gone either way as Caesar did have plenty of support.. and money), or go to war. Caesar clearly had ambition that exceeded the standard and faced with personal ruin and disgrace vs. the potential disaster that a civil war could cause the Roman state, Caesar obviously chose his own status above that of the eternal city. Despite this, he began to behave rather conciliatory, perhaps sensing the dire circumstances. After having essentially been declared war on by the Senate, he attempted to offer a compromise. First he asked to be allowed to maintain his governorship of Cisalpine Gaul and 2 legions, or Illyricum and only 1 legion, until such time as he could be elected Consul and enter Rome free from fear of prosecution. The Senate didn't act on this measure at all, though it was probably hotly debated. Without getting a response, Caesar decided to offer the same measure that already passed the Senate just a month earlier. Curio, in late December, offered that Caesar would agree to the original proposal that both he and Pompey dismiss their armies simultaneously. This time though, there was great debate and the Senate was divided on the issue. Whether certain members of Senate did not really desire peace, were prepared to risk war in order to defeat Caesar, or were naive enough to think that war could be averted by threatening Caesar is not entirely certain, but perhaps they simply felt that Rome would hold the loyalty of the people and that Pompey would ultimately crush Caesar if war could not be avoided.
On January 1, 49 BC and the days immediately following, the Senate rejected Caesar's final peace proposal and declared him hostis, or a public enemy. Caesar would have to give up his command completely or face war. The Tribunes attempted to block the measure through the people once again, but this time the Senate was entrenched. Much like Caesar, they too threw constitutionality to the wind and ignored the Tribunes using physical violence to stop their objections. Around the 10th of January 49 BC, word reached Caesar and he marched south with the 13th Legion from Ravenna towards the southern limit of Cisalpine Gaul's border. He likely arrived around January 11, and stopped on the northern bank of the small river border, the Rubicon.
Caesar seemed to contemplate the situation understandably for some time before making his final fateful decision. First testing the loyalty of his men, (he only had the 13th legion with him at this point) he gave a stirring speech pointing out the wrongs done to him (and the tribunes). With the clear support of his men Caesar added, "Even yet we may draw back; but once across that little bridge, and the whole issue is with the sword." He is then reported to have muttered the now infamous phrase, from the work of the poet Menander, "Alea iacta est", quoted as "Let the die be cast" or "Let the dice fly high." The Rubicon was crossed and Caesar officially invaded the legal border from his province into Italy, thus starting the civil war. Quickly taking several northern towns, the news reached Rome by January 17. Pompey, the Republic's hope, was left without his main army which was still in Spain, and his support base was in the eastern provinces. Despite having 2 legions to Caesar's 1, Caesar's Gallic legions were on the move to join him so Pompey and the rest of Caesar's opposition had little choice but to leave Rome immediately and abandon Italy to Caesar.