General Montero is initially Minister of War under President/Dictator Don Vincente Ribiera but revolts soon after coming to power. His reason? He thought Ribiera's government was in bed with foreign interests and diminishing the country's honor.
He's not exactly the sharpest or most cultured tack in the Costaguana box, but he's got confidence for miles (isn't that too often the case?). When he is still part of Ribiera's government, he gets ticked off wondering why no one is paying attention to him at a particular event (an occasion that marked his first time on a boat): "He was able to spell out the print of newspapers, and knew that he had performed the 'greatest military exploit of modern times' (I.8.51).
Yes, you read that correctly: he reads just well enough to know that people think he's awesome, which seems to be his sole argument for why he is awesome. In the absence of other evidence, we'll withhold judgment, Monty.
Despite his ludicrously over-inflated confidence, Montero and his forces are ultimately defeated, and he is executed.