What was the line from Dr. Strangelove? Mr President Ve VILL HAF a MINESHAFT disparity!
Seriously, why not just trot out the Neutron? That radiation will kill fifty or a hundred MILES into the ground and you can take over in a couple of days without even messing the floors in the bunkers.
In spite pf the hype, the neutron is no more than an increased radiation nuke. It would be less effective than conventional weapons in this application. You want force in this case.
The key words were "if they think". If they know we can make 50 + years of underground construction irrelevent, they'll come to the table with intentions other than buying construction time.