This is pedantic and unnecessary, but Caesar was undefeated militarily. His battle against Pompey was remarkable because of how much the odds were stacked against him. Most of the senate sided with Pompey because his victory seemed like a foregone conclusion.

Worth noting that Pompey's march was spurred at least in part by Caesar's ambition- Stalwarts of the Republic like Cato and Cicero supported Pompey as the lesser of two evils. In short there's a lot of nuance. (Caesar was also famous for being great to his soldiers, being apathetic about wealth, and inspiring incredible leadership)