Another rich source of data to consider is here https://ourworldindata.org/war-and-peace

I would argue that technology is changing wars such that the models of the past are irrelevant. We are entering the age of ultra-precision where making a weapon twice as accurate only requires 1/8 the destructive force to maintain the same lethality.  

In the future, the expanded use of things like drones coupled with facial recognition and other advanced sensors, will make war more personal but overall, less destructive. The goals of modern warfare are more about capitulation than killing. Killing is the result of forcing your will upon an adversary but it isn't the end goal and neither is the total destruction of an adversaries country. 

Nukes have never been used because there has never been a compelling use case while the reasons not to use them exist in abundance. For over 70 years, nukes as a deterrence has prevented another world scale war.

What this chart suggests is that currently, the world overall, is in the most peaceful extended period seen in over 600 years. Global warfare changed forever in 1945.

The pre nuclear modern warfare trend line is an expression of the industrialization of killing. In a nuclear free world, we would have made killing more and more efficient making the horrors of the battlefield truly unbelievable.

Yet that is not quite as chilling as the prospect of nuclear war. Conventional wars would have likely occurred on the global scale at least once since the end of WW2. The use of chemical weapons and bio weapons would have likely been in the mix. The use of such weapons is muted today as such large-scale use would warrant a nuclear response. Without nukes there is no such direct threat discouraging their use.

I think you missed an opportunity to quantify targeting in US military nuclear planning. Now while the details of specific weapons allocations on specific targets is classified, the process of determining force allocation on legal targets is not.   The US has made it clear that we do not target cities since the early 1960's. since that time, the rules that govern the use of nukes in the us military have grown to be very well defined.

Under US military law, weapon choice must match the scale of the threat. In other words, you don't use a nuke on terrorists in Afghanistan. The weapon cannot be used to target civilians, civilian infrastructure, historical or cultural areas of importance or monuments and museums, The weapons cannot be used against the environment like destroying a huge dam or targeting a nuclear power plant. Additionally, a weapon must be chosen that minimizes civilian casualties while still delivering the outcome that is the justified military objective. So, if a target can be taken out with conventional weapons to minimize collateral damage and civilian deaths, then conventional weapons must only be used.

In areas where civilians are regularly associated with, the military must give advance notice of their planned attack so civilians can vacate the area. Further, any use of such weapons cannot be in retaliation or revenge, you cannot take out a city in response to an attack on one of your cities, no tit for tat. You are also not allowed to start a war of aggression such as invading another country in an effort to subjugate the land and its people.

These rules hold true even if the President orders the use of nukes in violation of these requirements under military law. Such an order would be illegal and the Military would not follow an illegal order.

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Further guidelines exist under the US Nuclear Declaratory Policy, which further restricts their use in more general terms, stating nukes will never be used against a non-nuclear adversary. However, such statements are left intentionally vague. The US maintains a policy of “calculated ambiguity”. While the US seeks to signal its intent clearly enough to highlight “red lines,” or situations in which the United States may consider employing nuclear weapons. Yet, it also refrains from telegraphing the type and size of an attack an opponent should expect, should it choose not to be deterred.

This ambiguity seems to be in conflict with military law at times. The threats and innuendos that exist between politicians and states are filled with more rhetoric than policy and law. Something the media likes to fan the flames under. No matter what is being said in the media, nothing said overrides the military law on the use of weapons. Politicians talk a lot of smack at the end of the day, keeping a grounded perspective is very useful and necessarily warranted.

So, when is it legal for the US to use nukes?…

Targeting an adversary's nukes and nuclear support and production facilities not near population centers and some military bases that are also outside of civilian centers. In certain battlefield scenarios that were envisioned during the cold war when Russia could have invaded western Europe with overwhelming force using thousands of tanks. A situation that no longer exists or is even possible. They can be used to destroy or damage deep hardened underground military targets where conventional weapons would not do, such as weapon production facilities in North Korea.

Smaller nukes, like the new W76–2, a 5kT warhead that was recently added to our strategic SLBM fleet or the lowest yield on the B61–12 at only 1/3 a kT, blur the lines between nukes and conventional weapons. Such weapons could be legally used as airbursts over large military facilities in civilian population centers with minimal civilian casualties. The concern here is that it becomes too easy to use nukes and that line can be crossed with small nukes and not larger nukes. To any adversary, any attack on them using any nukes, no matter how small, changes their playbook dramatically.

Anything else, with few exceptions, would be illegal under US military law.

This is why the Military considers nukes largely useless which is the primary reason they have not been used since WWII. Their role today is 99.99% deterrence and .01% functionality.

Generally speaking, any effort to improve the utility of nukes, such as developing lower yield warheads, is controversial. While it seems like something you would try to do, make a weapon more useful. In the case of nukes, that change in utility detracts from their primary role of deterrence. Nukes must be viewed from their strategic role more so than any potential tactical usefulness to prevent the temptation of their use in future combat scenarios.