Nukes have been civilization ending since the 1960s. You should see the plans made by American planners at the time. They joked about having to turn the nation of Albania into nothing. US has enough sensors to track when launches occur that a retaliatory strike would be launched almost immediately. Even if missiles somehow got through to the continent, we have many of them in discrete locations on submarines, waiting to launch. We are even using our navy more for missile defense since there are advantages to being mobile and ephemeral than having a fixed defensive battery that everyone can see on google earth satellite imagery.