Vox: If the military won't defend the border, then they need not exist.

Absolutely correct.

The original purpose of the US military is to defend the borders of the United States from all foreign invaders, and that doesn't just include foreign uniformed military services.

If the military can't or won't fulfill that purpose, or if the politicians can't or won't order the military to do it, then simply disband it altogether, allow Americans to arm themselves as they please, and let the balloon go up once and for all.