...Also having our nuclear force in submarines is desirable because the subs would be far from our population centers. If someone did try to destroy our nuclear weapons capability, at least we would have the option of responding with most of our population left alive. This concept of keeping the nukes far from our population is the main reason why most of our Minute Man III silos were located in North Dakota.

A "first strike" to try and destroy our weapons capability is something only a major nuclear force, like Russia, would be able to do, and I don't think even they could do it any more. Even without missile defense, it is a tactic that would involve hundreds of missiles, and is NOT compatible with the "terrorist sneaking a bomb in" scenario. So, we have to establish right off what we are most worried about: large scale missile attacks, one or two missiles, or someone sneaking in a nuclear packing crate. Or maybe all of the above? Then we can discuss specific defenses.

By the way, the actual reason that missile sites are located in the center of the continent is to increase to the maximum the flight time of a sub-launched enemy missile, giving the silo boys and the rest of our defenses more warning time.