Re: Or what if China, Russia, or Japan invaded the US or Canada?
We would probably risk a nuclear free WW3 rather than an all out nuke showdown. Every country knows that every country will lose when nukes are used. The only to gain an advantage would be to somehow develop a missile defense system so advanced, that it would be almost impossible to gain a strike against a country without taking out the defense system first.
Russia? It reached it's peak in power when it was the Soviet Union. It's losing citizens to immigration. They would still be a formidable foe to the US but a united Europe would crush Russia. Only if no one else intervened and it was Russia vs US one on one would they stand a chance. The US has too many allies with NATO and all.
China? You bet. They're becoming more modern every year. In turn, they're becoming more Democratic as well. Reasons for war with the US would probably be over resources rather than ideology.
Japan is the US's bitch. I for one dream of the day when Japan can have it's full military back. When they don't need us to protect them. Japan's self defense force would be rolled over by the North Korean Army even though Japan is far more advanced. If a world war would ever be inevitable, we NEED Japan as an ally. We need them to be self sufficient militarily. We can't babysit them forever. WW2 ended 67 YEARS AGO!