Pompey and his allies induced the Senate to demand Caesar give up his provinces and armies in the opening days of 49 BC. Caesar refused and instead marched on Rome.
Source: Wikipedia
Addendum:
The civil war ultimately led to Caesar’s becoming dictator for life (dictator perpetuo). Caesar had been appointed to a governorshipover a region that ranged from southern Gaulto Illyricum. As his term of governorship ended, the Senate ordered him to disband his army and return to Rome. As it was illegal to bring armies into the northern border of which was marked by the river Rubicon, his crossing the river under arms amounted to insurrection, treason, and a declaration of war on the state. According to some authors, he uttered the phrase iacta alea est (“the die is cast”) before crossing.
Source: Wikipedia
That’ll show him!