Think for a minute what it would take to invade and conquer a nation of 350 million people, 3.8 million square miles. How many men would that take? How many ships and airplanes, fighters and bombers and missiles and bombs and bullets? And even if you had that kind of force, what would be left when you were done? How many TRILLIONS of dollars would it take? Mind you, this is a nation with huge oceans on either side so you have to go at least 3,000 miles even just to get there! Further for the Chinese and Russians.
So what you're asking is like 'What would happen if the moon fell into the Pacific Ocean?' or 'What would happen if the sun exploded?' It's seen as just so unlikely that we don't worry about it, we don't have contingency plans for it.
All through the Cold War we were reminded that both the USSR and China had enormous advantages in manpower. Which they sure did, but wars aren't won by manpower anymore. Our military isn't based on swarms of dogfaces with rifles. Almost our entire war-making capability these days is based on air power. We pick fights only with tiny 4th rate nations, and then we bomb them to a pulp before sending in ground troops to 'clean up'. This is because the American People only care about a war when American casualties get up to a certain number, and while aerial bombardment is the most expensive way to fight a war, it has the least US casualties.
It's hard to imagine an all-out war today between major powers without the use of nukes. Remember the Colin Powell Doctrine? (Don't be embarrassed--nobody does!) That was the policy in effect at the start of GW Bush's wars in Afghanistan and Iraq. Hit them with overwhelming force, use the most powerful weapons at our disposal to get the war over quickly and save lives. And never go into war without an exit strategy. Bush tried that and it didn't work, so he abandoned it.
But this is why we don't pick fights with other kids our own size. Why we never attacked the USSR or China, or North Korea once they got nukes. It's why Iran wants nukes so badly now. Everyone in the world (except, apparently, Republicans) understands that nobody wins a nuclear war.