It’s worth acknowledging that superior firepower hasn’t directly resulted in success for the US military. Guerilla armies like in Vietnam and in many countries in the Middle East such as Afghanistan have repelled US invasion, and wargame simulations (including US on US wargames!) have repeatedly shown this.

Add into the mix that there was a serious degree of military revolt (complemented by homeland revolt) in the Vietnam War (and other later wars too). While this was largely boosted by it being a conscription army at the time, I still believe an internal US war would make the military similarly vulnerable to internal conflict, and even external sabotage by citizens.