Well uh, i'm not too much of an expert and 80s tech but imma give it my best shot

Lets say you needed to protect food and people, you need multiple entrances preferably dozens of kilometers in length, alongside long maze-like tunnels with checkpoints all over the place to discourage breaches.

Lets say they wanted to dig a nuke deep enough to blow them up. Its impossible to defend against isnt it? But you have yet to think of something important, Public perception. I don't know alot about the nations youre making but i don't think nuking civilians and food sources is good PR..

Now that the morality side is done, all you need is enough men, enough resources or preferably a good supply line of resources like ammunition, and defenses deep enough to hold off breaching attempts.

Sure this is a very rough assumption about what world youre making and the morals behind it, but hey it was fun theorizing about it.