Earlier quoted context omitted.
The US (with its support of the military) has been at war in the Middle East for nearly 2 decades now with insurgents. The argument is not that a rebellious citizenry will necessarily win a war, it's that it will draw out a bloody civil war so long and so expensive as to be a form of mutually assured destruction, the risk of which acts as a check in and of itself.
The fact is that the dictator would still win. The rebellious citizenry would live a life of absolute misery, just as those in the middle east do. The 2nd amendment made a lot of sense when weaponry consisted of horses and rifles, not computer-guided missiles. If there was ever a true US dictator, the 2nd amendment would mostly be used by the oppressed to rob, attack, and oppress one another.
As such, dictatorship and imposition of rule through force has to be considered in light of other political options. Oppressive government does not generally start with an all out war to subdue the populace, using strategic weapons like missiles and bombers. Insurrections are managed with lighter arms not because totally destroying the enemy is not a military option but rather because it is not consonant with the relevant political goals.
The right to bear arms isn't about what you do for all out war -- that's when you move from citizen soldiers to building armies -- it's about trimming the distribution at the lower end and improving the odds of the citizenry being able to make it up the food chain in a reasonable amount of time. (Knowing what guns are actually called is a surprising advantage there.) Even Hitler started small, and disarmed the citizenry early.