"you and a dozen of your friends are going to overpower the US military"
Especially since the 2nd Amendment was written when there was no US military at all. The discussion about a professional standing Army went on for a while, as it was exactly seen something an authoritarian government could use to subjugate the population. For years, US didn't have a Navy at all but a few small ships.
The fact they defeated English troops couldn't ignore UK couldn't send in those time a powerful force enough without risking to be attacked and badly defeated in Europe. A local standing Army would have not such a issue. So "militiae" - well regulated - looked a good compromise.
Later the situation wholly changed - as battlefields became more complex, and weapons too, and weapons became increasingly designed for battlefield use specifically, not generic ones for hunting/self-defence/combat ones, requiring specific training for efficient use. Ammunition too was increasingly designed to do much more damage on impact.
Now you've got tanks and airplanes...