The same people that claim the US military is by far the greatest in the world and can't be defeated, are the very same people that will tell you they could defeat the US military with only semi-automatic rifles and 'true grit'.
Uh huh.
Your argument presupposes that it would be necessary to fight the greatest military in the world, and that the U.S. military would long remain the greatest military in the world. Both scenarios would be unlikely.