Defense is impossible. Missile defenses are meaningless except as money funnels to the arms industry. This is not the place to go into decoys, hypersonics, Poseidon, maneuvering glide vehicles, bastion stationing, MIRV, just plain boring old cruise missiles, and so on. Coastal cities are particularly easy targets, being vulnerable to submarine-launched sea-skimming missiles. Washington, New York, Boston, San Diego, Los Angeles, San Francisco, Seattle for starters, all gone.
A modern country is a system of systems of systems, interdependent and interconnected—water, electricity, manufacturing, energy, telecommunications, transportation, pipelines, and complex supply chains. These are interconnected, interdependent, and rely on large numbers of trained people showing up for work. Modern warheads are not the popgun squibs of Hiroshima. Talking of repair any time soon after the nuclear bombing of a conurbation is foolish because the city would have many hundreds of thousand of dead, housing destroyed, massive fires, horrendously burned people with no hope of medical care, and in general populations too focused on staying alive to worry about abstractions like supply chains.