China cannot invade the United States. They do not have anywhere near the capability to project that kind of power. Technically their navy has more ships than ours but most of them cannot go more than 1,500 miles from their own shores. They have no practical ability to project power to the US Coast.Who knows. Most people think so but it could drag on for awhile. I have no idea.
Russia cannot invade us. They can nuke us to oblivion but they don't have the resources to invade.
China can. China can invade us or at least try to.
Ww3 can last 1 hours and be over, or drag on and last 10+ years. I just don't know. Way too many unknowns.