Julius Caesar's protegé was Marcus Junius Brutus, who later would become Caesar's assassin along with Gauis Cassius Longinus, another former Pompeian.
Caesar had a good relation with Pompeii until he became governor of Gaul (roman province in Western Europe inhabited by Celtic tribes).
The tribes did not easily accept his rule easily, however. After the Gallic Wars, waged by him against several Gallic tribes, he finally established his rule after defeating Vercingetorix (celtic tribe leader) who tried to unite all tribes against the Roman dominance.
Gaul was of utmost military importance to the Roman Republic. Such as its dominance granted Caesar his position as sole ruler of the Roman Republic.
He then led all of Gaul, except 3 tribes to rise up against Pompeii, which was a protagonist of a political quarrel against Caesar, point from which the conflict between him and Pompeei started.