But you have to wonder what the point of an attack like that would be. Who wants to take over an entirely destroyed country? What would the purpose be?
The US is a mass of land rich in Natural resources we won't use for ourselves. Do to restrictions, regulations and politics we can't or won't look for our own oil, coal, gas, fresh water etc. An invading occupying force isn't going to care about EPA and OSHA regulations. They will take what they want how they want to.