The conquest of Gaul saw Julius Caesar at the head of a large and seasoned army, and in 49 BC, he led it across the Rubicon into Italy. It was an act of war, since no commander was allowed to take soldiers outside his province without express senatorial permission, and the River Rubicon was the boundary of Cisalpine Gaul. Caesar marched south to occupy Rome, while the senatorial party opposed to him fled across the Adriatic to Dyrrhachium. There they assembled their own army under the command of Pompey, who was now Caesar's arch-rival. Caesar followed, and lad siege to Dyrrhachium. The two armies met at Pharsalus in Thrace, where on 6 June 48 BC, Caesar won an overwhelming victory.
Pompey fled to Egypt, where he was treacherously murdered, but this did not mark the end of resistance to Caesar. Late in 48 BC, Caesar sailed for Egypt and he defeated the ruling monarch in the Alexandrian War and placed Cleopatra in control. In 47 BC, he marched his armies back to Italy through the eastern provinces. The survivors of Pharsalus regrouped in North Africa, but in 46 BC, Caesar won a further victory against them at Thapsus. The last sparks of opposition were stamped out in 45 BC when Caesar defeated the army of Pompey's sons at Munda in Spain.
The victory at Munda removed the last of Caesar's enemies in the provinces. Senatorial opposition to the rule of one man was still deeply entrenched, however, and came to a head in February 44 BC when Caesar had himself appointed perpetual dicator, making him in effect the monarch of Rome. A month later, on 15 March, he was assassinated by a group of senators on the eve of his departure for a campaign against the Parthians.