Tolik & Roadracer are right. The logistics of an actual land invasion of the US, Russia, or China are near impossible in a conventional war kind of way. Cyber war is much faster and would do far more damage, especially if combined with some basic up front and personal jihadi terrorism. Nukes are too messy for anyone to truly win. Though I put my money on cyber war, I don’t discount biowar entirely. China has far more people that they need or want and if they lost a few hundred million to biowar, it would be an added bonus. Before such a thing was unleashed, there would be an vaccine that the chosen will have already received.