The Civil War in the eyes of most people is not glorious, but rather one of the worst crimes you could possibly commit when the state is all-important. Only under the most extreme circumstances should one be allowed to (in the eyes of the people that is) begin a Civil War with just cause. Caesar took this into consideration, but too many things were going wrong in Rome for him not to begin the war.
The first of many problems was the collapse of the Triumvirate. The Triumvirate was one of the main parts of the government of Rome, with which there were three leaders, which at the time were Caesar, Pompey, and Crassus. This was never truly working all that great, but held itself together by the marriage of Caesar's daughter Julia, to Pompey, and the friendship Caesar and Crassus shared. But, all this came to an end when Crassus was killed in a battle against a Parthian army. Then, not too long afterwards, Julia was murdered by someone who had broken into her home. This, destroyed the bond between Caesar and Pompey, and made them drift apart. Caesar seeing all this taking place, attempted to restore the bond by proposing to Pompey's only daughter, but was not allowed to by Pompey.
To only make matters worse, Rome was slowly slipping into total anarchy. The government was becoming corrupt with bribery. The elections were being stopped, and there wasn't a consul elected in 53 or 52 B.C. Most authority was lost, the streets became rioted, and unsafe. During this time, Pompey tried to annul the Law of Ten Tribunes without notifying Caesar. If this happen, it would of removed a lot of Caesar's power. Caesar saw what he was trying to do, and stopped it before this action took place, and now knew for sure that Pompey was no longer his ally, but instead an enemy.
Pompey tried another devious act against Caesar, which this time worked. He had the senate pass a law that made Pompey and Caesar both give up troops, and send them to the East, where they were supposedly needed against the Parthians. This seemed fair, but it made Caesar lose two legions, one that was lent to him by Pompey in the Gallic Wars, and one of his own. Once they were positioned there, Pompey decided they were no longer needed, and sent them to Capua (a city in Rome) under his command.
The final event that drove Caesar to rebel was the Senate declaring Caesar to be a public enemy. They said that he was doing harm to Rome, and was a threat to the entire country. They declared that the troops stationed in Capua go and defend the capital against Caesar.
Caesar at the time was stationed in Cisalpine Gaul, with only one Roman legion of troops, and some small German and Gallic cavalry detachments. When he learned of what the senate had done, he knew he had to act immediately. He sent his troops to the bank of the Rubicon, which was the river that separated Cisalpine from Italy. This was the spot, should any army cross, it would be considered an act of war, and the start of a Civil War. He knew he had to act now, because Pompey only had two trained legions under his command. Both legions had served under him in the Gallic Wars so they would more than likely desert than fight against him. All the other armies that were being set up were new and untrained, and probably wouldn't fight well, if at all. And so, he crossed the Rubicon, knowing at this point there was no turning back.
Things actually turned out to be better than expected. He marched down the coast conquering town after town not only without a fight, but in fact gaining support of the towns, and increasing the size of his army. Even the troops recruited by Pompey himself joined Caesar's side, and went off to fight Pompey.
The only resistance Caesar ran into was at Corfinium, at which one of Pompey's commanders had made a stand. The siege lasted only a few days before the majority of the troops mutinied and joined Caesar. The swift victory scared Pompey and his...