Now let's say you don't have conventional weapons.

China invades all South East Asia progressively. Would you nuke them? Russia invades Eastern Europe. Would you nuke them? If yes why hasn't the US nuked Russia yet?

Then all your allies suddenly switch side after being overthrown by dictators who are in favor of your enemies. Would you nuke them?

Another one: you are terrorists associations preparing attacks against you in camps all across middle east. Who do you nuke to defend yourself?

Conventional weapons are actually the only useful tools nowadays, because war is much more sneaky than it used to be, and it is much more about fighting for influence than really invading countries or defending you territory.

On the other hand let's say all had the bomb but are reluctant to use it. The nation's fight it out with conventional weaponry until the axis powers, realizing they have lost and with invasion in progress finally feel they have no other choice and commit mutually assured destruction. I think it would be hard to claim that wouldn't have happened given the temperament of some of their leaders. Perhaps in this case the allies would have showed restraint and not invaded to end the war - perhaps they would have been willing to negotiate terms.

I think basically my idea is a terrible one. But I also think the world war II scenario would end in nuclear exchange - the only way to avoid that is not push any power to the point of last resort, and negotiate an unsatisfying agreement that leaves the aggressors in power, no matter what horrible things they do. Would all have shown such restraint, would cooler heads have prevailed amid the terrible atrocities of total war? I'm not particularly optimistic of the chances.

What if the invaders are non-nuclear armed?

There is a wide range of responses between nothing and total nuclear annihilation, even without world police status.

You deliver an ultimatum, wait for the invader to withdraw. If they call your bluff, you level them. Just a city at a time if they don't have nukes. Wholly if they do. I mean if your going to rely on a nuclear self defense, when someone attacks you, you have to be willing to use it.

Sometimes doing nothing is the best response. I didn't say it never was. I just said you don't want that or nuclear war to be your only options.

> What business is of the US what happens in the rest of the world?

Do you use anything not made in the US? If you do, then you care what happens in the rest of the world. Even if you personally don't, enough other people in the US do that you need to care what happens in the rest of the world, because you depend on those other people in the US and they use things from the rest of the world. The world is highly interconnected.