"I don't know what the 3rd world war will be fought with, but the fourth one will be fought with sticks and stones" (some guy who did something with "relativity")
However, I don't know if I fully agree with your sentiment. Yes, it is true that leaders of dictatorships love to show their "strength" by threatening everyone with their instant death button. But at the end of the day nobody wants a nuke on their head. If they start using a nuke, others will respond in kind. Dictators are cowards who want the rule and not the responsibilities that come with it.
You have a lot more faith in humanity than I do, then. All it takes is one person to set off a chain reaction of these things...or one person to end it, depending on where the battle lines are drawn. I realise there is a huge difference between words and action, but war is not the best of times. If we're officially in a World War, do you really think Trump or Kim, as two very prominent and highly volatile examples, will show any kind of restraint? I would argue that they would be
required to fire missiles in a wartime setting to display that "strength" and justify the people's faith in them. War is when threats stop being threats and start becoming action.
Nobody wants to start launching nukes after the last time, but nobody really wants a war either, and the quickest way to end a war is to just obliterate the opposition before they obliterate you. I would say the threat of a nuclear war is what stops World War III from becoming a reality - otherwise the US would have gone to war with North Korea and China at least by now - but if we've already moved past that point and are in World War III, the gloves are going to come off. Someone will do it because they'll be afraid that either a) someone else will do it first, or b) they'll be removed from office for someone who will, because that will be what the majority of the population want.
The repercussions will be enormous, but it'll all get swept under the rug like everything always does. They'll have wiped out hundreds of thousands of enemy combatants, and in a few days/weeks there will be "evidence" that the enemy had planned to do exactly the same thing, whether that would be true or not. People only care about results, and the end result will be that the war will probably be over and they'll still be alive. I think as a species we've more than demonstrated by this point that the environment is a secondary concern to our own wellbeing, so the consequences of launching nuclear missiles would be a problem for the next generation to figure out.
If we're assuming that neither Trump nor Kim are in power I still don't think it makes much difference - North Korea are essentially still in a World War II state of mind, and that won't change anytime soon. Russia and China will always be objects of suspicion to the West as well for various reasons: Russia is already engaging in cyber terrorism to some extent (to what extent depends on how big a threat you believe Russia is, because they are keeping quiet about it to bolster their reputation as a world power) and China has the largest military force in the world, amongst other things.
tl;dr I don't think you can really apply peacetime logic to a wartime setting, because only in the complete absence of logic and reason - and sanity in general - can war really happen. Because we've seen the kind of destruction and tragedy that war can cause, and we all know that another war would be far more devastating because of the advances in science and technology.
