In January 49 BC, after being declared an enemy of the Republic, Julius Caesar led a single legion south over the Rubicon from Cisalpine Gaul to Italy to make his way to Rome. In doing so, he deliberately broke the law on imperium and made armed conflict inevitable.
As the Senate didn't know that Caesar only had a single legion at disposal, Pompey and the rest of senators fled the City to regroup and allow more time to rally loyal troops and raise more legions.
My question is : how on earth did the Senate not anticipate Caesar marching down on Rome ? To me, it seems that when you call someone who possesses a dozen legions an enemy of the Republic, it basically forces him to choose a civil war rather than face prosecution. It's not like a politican marching on Rome with an army had no historical precedence – Sulla did it only half a century earlier.
So why didn't the Senate anticipate Caesar's response by say, raising legions or at least mobilising troops from Greece into Italy ?
This is a good question. The simplest answer to it is that Pompey and the Senate didn't expect Caesar to march on Rome yet.
(A note here on sources. I'll be leaning heavily on Suetonius and Plutarch, who are some of our best primary sources for the life of Caesar.)
The Senate certainly understood that they were driving Caesar to declare war. This was arguably the intention. While there were some senators, like Marcus Cicero, who tried to brook a compromise, Caesar's opponents in the Senate had the numbers or tactics to enforce their will and drive Caesar to war, which they did. From their perspective, either Caesar would acquiesce, in which case they would destroy him civilly, or he would declare war on the state and be destroyed martially.
When we look back at the war, we can sometimes take the actual outcome as the inevitable one. Caesar won, therefore Caesar was always going to win. But for most of the Civil War and especially in the period leading up to it, Caesar was the underdog. He had fewer men, worse supplies, and fewer supporters. Some of his most capable lieutenants defected to the Senatorial camp, notably Labienus, a skilled and gifted general. Moreover, while Caesar was probably the best general of a period of great generals, it was still a period of great generals! Pompey may have been old, but he was still sharp. It is not hard to imagine the many ways that Caesar could have lost the war, all the way until the very end. Part of the reason the battle of Pharsalus occurred was because the senators in Pompey's camp deemed Caesar such easy prey that they goaded Pompey into attacking against his better judgement, which was to starve Caesar out.
I go into all of this to get some idea of why Caesar marching was so unexpected. Caesar was certainly preparing for civil war and the Senate expected as much, but when he marched into Italy he had with him only a single, undermanned legion. Plutarch tells us that even that was outnumbered by the forces that Pompey had on hand. We're told that poor information and general terror was what prompted the flight from Rome, but in truth it may have had something to do with wanting to avoid a defeat before it was necessary. Holding Italy was good, but it was better for the Senatorial forces to regroup in force. It takes time to assemble an army, and perhaps if Caesar had taken the time to assemble his, the Senate would've had theirs assembled as well. But instead, it was Caesar's speed and daring that caught them off guard.