Julius Caesar was able to march his army into Rome because he had support from the people.
When his political governorship in the Gaul and Ilyricum expired, the Roman Senate ordered Caesar to disband his army and return to Rome. By this time, he had already become a hero for the common people and had proposed several political reforms. Thus, when he crossed the Rubicon, the common people supported him and approved his actions.
Caesar crossing the Rubicon river with his army (a capital crime according to Roman law) precipitated the Roman Civil War, leading to him becoming emperor-dictator for life.
