What would be the key differences between real-world nuclear warfare and alternate reality where each nation has only a single thermonuclear weapon?

Question

Consider an Earth-like world where every nation in the world agreed to dismantle and divide all nuclear arsenals so that every nation was allowed only a single 50 megaton thermonuclear bomb (equipped with an ICBM delivery system), and all production facilities destroyed except a single very small facility at a "World Alliance Headquarters" in order to replace broken weapons or produce for new nations in case new countries are developed. If someone uses their nuke, it will be replaced only after a period of 10 years.

The members of the agreement decide on a very thorough annual, international investigation aimed at discovering any illegal nuclear facility, manned by investigators from as many countries as wish to participate. These investigations are regulated so that the governments are allowed to hide any classified small documents and even large weapon or vehicle prototypes from sight, just not conceal any areas large enough to house components of nuclear production facilities.

Also: Someone mentioned the systems would be useless due to interceptor missiles - To counter that, let's assume that as part of this agreement, all long range missiles (greater than a few miles) except for the single allowed nuke are banned. Otherwise the countries could just amass tens of thousands of non-nuclear missiles and have the same effect as a nuclear arsenal.

What would be the key fundamental differences between real-world nuclear warfare and this scenario?

About the broadness of the question: I'll specify that I'm asking about only the key, fundamental differences between a real-world nuclear war, not "how it would play out" in general.

Edit for more situational detail, if desired:

While the nations may be able to get away with covertly working to develop a larger nuclear arsenal, punishment for breaking the agreement includes a 10 year denuclearized period (not even the 1 nuke) and a set of harsh, pre-determined, globally enforced sanctions for the same period of time. Further attempts to re-nuclearize outside of the agreement are considered an act of war against this "world alliance" and some form of removal of the leadership with replacement based on the existing government's traditional policies as if the leadership were dead. Let's assume, as a premise for the question, that this is enough to dissuade nations from breaking the agreement.

Answer 1

One consequence would be to make alliances much more important from a nuclear deterrence perspective than they are now. If “you can fire mine if I can fire yours” type agreements are allowed? If so, it would give both nations a guaranteed second strike capability. It might also provide some impetus to create separate allied / united / confederated states rather than large single nation states depending on the exact rules.

The exact definitions of statehood and who exactly was allowed to have weapons might become a hugely contentious international issue. For example at what point of integration does Europe become one state and lose its multiple nuclear capability (Brexit and French politics aside).

Another consequence would be an increase in the size of conventional armed forces because if there was no Mutually Assured Destruction then a nuclear war might be “winnable” especially if you are a dictator, you rule a large country and you have some anti-ballistic missile technology.

Many countries might well opt not to have nuclear weapons even if there was no cost on moral grounds or on the more pragmatic grounds that it would make them less of a target. Nations might be less keen to use their one and only weapon against a non-nuclear state when so many were armed with nuclear weapons. But a significant number of new nuclear powers would undoubtedly be formed overnight changing the world nuclear balance of power.

Militarily the number of missile launch sites and platforms might still remain quite high to provide the maximum uncertainty effect over where the real weapon was to ensure a second strike capability. Conventional anti-ballistic missile technology would also receive a boost as it would enhance the prospect of having a second strike potential.

Answer 2

My take on this is that the nukes would be mostly irrelevant in a full on war, any large nation that could afford a large military could take a hit, alliances of many countries are more troublesome.

Btw, is delibery mechanisms included? If not nations that can not afford to develop ICBMs are at a dissadvantage.

An interesting problem with this development is what to do when nukes are reported stolen. If I can just give a warhead to my terrorist friends (maybe with some agents keeping track of them) with a prommise to wait a couple of years before detonating in a car bomb in a rival capital then that might become a common tactic. If the nation that does this soon gets a new bomb this can really get out of hand. Similar problems occure when militaries covertly capture bombs from others. That could become a trend as well

Answer 3

Alliances are important, though I wouldn't put too much stock in them ... OP never said that once you use your nuke you get a replacement! If this assumption bears out, every country gets one and only one shot at nuking someone. If you are the ruler of a country, are you really going to use your one-shot to help your alliance partner, leaving yourself vulnerable thereafter? More likely you'll save it for your Captain Ahab option:

To the last, I grapple with thee; From Hell's heart, I stab at thee; For hate's sake, I spit my last breath at thee.

I think the more important factor is going to be size and dispersal. Countries which are big enough to absorb a nuke (or two, in case someone actually does respect his treaty obligations) are going to have a gigantic advantage over smaller, more concentrated countries. Everyone will know this.

Expect that many countries will be nukewise paralyzed by the notion that there could be a worse crisis later. The bombs would be stored away for a perhaps-hypothetical existential threat. I expect that border wars would continue, largely pushed by the big dogs. What you won't see much of -- not that you see much of it in the past 60-odd years anyway -- is wars of total conquest.

Answer 4

The primary difference is that if one country wants to go to war with another, it would first seek to neutralize or hijack the one missile of its opponent. Because once the single missile is used up, the country has no deterrent to prevent attack by another country.

Countries would have a lot of options there. This would put a high premium on espionage and suborning rebellion. Also simple terrorism. If you find the enemy's missile, you can blow it up with a conventional bomb. Or steal the nuclear material out of the warhead. Infiltrate troops and take possession of the controls. Destroy the launch site. Knock over the missile. Replace the control mechanism with your own.

There are reasons why countries don't just build one missile. As others have noted, a geographically large country could simply afford to take the losses. And a single missile is more subject to hijack or sabotage.

Countries might form into shared-defense groups for that reason. Because it's much harder to hijack/sabotage missiles in five, ten, twenty, or more countries such that they are all sabotaged at once.

Once the single missile is eliminated, war would become more normal. Conventional weapons are conventional weapons. The lack of long range missiles would create more of a need for mobile platforms like aircraft carriers. The ability to create forward bases would also be very important. And those two things would make logistics even more important.

If a country still has its missile and is attacked, the choice of targets may not be clear. Do they pick the capital city? The biggest city (if different)? The city with the most military production? The armed forces themselves? Do they save the single missile in case they feel the need to deter someone else's attacks?