Some people in Rome were afraid of Caesar's power.
In 49 BC Caesar and his army set out for Rome, where he intended to be elected consul again.
The senate ordered Caesar to come without his army. Caesar refused. He crossed the Rubicon river (the border between Gaul and Italy) and entered Italy with his army.
The senate declared that Caesar was now an enemy of Rome. This marked the start of a civil war.
With his army Caesar defeated all his enemies (mostly senators) and chased them across the empire, even in Egypt. Here he fell in love with Cleopatra, sister of the pharaoh.
Back in Rome he was now the only ruler. He allowed the Senate to remain, but he did not want to share his power with the senators.