In a conventional war, the US is too big for any nation (and maybe every nation) on earth to attack with a force that would overwhelm.

In a nuclear war, Russia has enough missiles to cause major destruction to every major and most medium cities in the US. That level of attack would leave all but the most rural/specific areas in the country habitable for the next few years.

There is absolutely no chance that a missile launch from a russian sub, that is allowed to sit 20 miles from the coast according to the maritime borders, would be stopped anywhere on the coast. We don't have some amazing missile defense system that sits active across our whole coast line that could stop it. We don't even have our patriot batteries deployed almost anywhere that would be a deterrent help.

You can bet that will be a priority soon though....