Julius Caesar may have been the largest influencer in Roman history. He was elected governor of Spain at a young age yet he still wept after seeing a statue of Alexander the Great, because he felt that he had not done nearly as much as Alexander. He was very popular with the people, but he worked mostly away from Rome. His soldiers were loyal to the bone.
The first triumvirate was a combination of Pompey's military power, Julius Caesar's popular support and Crasus' wealth. They had a mutual agreement to help each other, each providing each others with what they could not provide for themselves. Crasus died of natural causes quite soon after the formation of this pact and Caesar started to gain military power.
Pompey sent a letter to Caesar, asking him to come to Rome. If Caesar came with his army he would have to fight. If he did not come he would be arrested. He had no choice but to come to Rome. He crosses the Rubicon river and defeats Pompey and the senate who flee to Greece, once they fight again Julius Caesar destroys them killing the majority of the Senate.