Their constitution says the US isn't even supposed to keep a standing army. There's lots of discussion about that by the people who wrote it and decided on the specific language.
Also, you're right about the 2nd amendment. The part about the well regulated militia is part of the right, a qualifier, not subsequent.
But, people in the US, both government and citizens, clearly don't follow their constitution very well. It's used to justify whatever they want to do when they can and freely ignored when inconvenient.