Can you imagine the size of the invading army to occupy all of the US and Canada? (Oh, and Australia, New Zealand, Japan, South Korea, Philippines, Thailand, Turkey, Saudi Arabia and essentially all of the European Union). China and Russia don't have blue water navies. They have modern submarine fleets. Neither of them are as large as the American one. But even if they were, a submarine fleet isn't sufficient to invade the most powerful nation in the planet.