It is my understanding, and correct me if I’m wrong, that all able bodied men who are citizens of the US are legally defined as militia. And it is that law which allows for draft in military service.
It is also my understanding that the 2nd was written in the way it was with the understanding that eventually our government would devolved into totalitarianism of one sort or another, and it was the duty of we, the citizens, to take up arms against it.