I’d put ten bucks on it being the USA.
Israel is very canny, and while they live on the pretense that they are “all alone” despite the massive amount of military aid and intelligence they get from the US and Britain, they are well aware that a unilateral nuclear strike against another nation would cut all support.
Iran, despite having nuclear pretensions and being run by a bunch of religious assholes, would have little to gain by launching a nuclear strike against another nation. Iran is likely to enter the nuclear arena and rattle sabers, but actually initiating an attack is unlikely.
North Korea is a contender, but I honestly doubt they have the resources to weaponize and deliver an effective nuclear weapon. I find it far more likely that Pakistan will either launch an attack or will provide a weapon to some part that does. The odds of this happening some time in the next ten years are not insignificant.
Stranger
You’ve posted this scenario in several threads. It’s utter madness to think that the UK would choose to become an international pariah over the Falklands.
More to the point, what the hell would that accomplish? At the end of the day, the UK would still have no way logistically to hold the Falkland Islands or fend of the now furious Argentines who would at that point rather burn the islands to the waterline.
Stranger
If for some bizarre reason N. Korea wanted to blow up a major city and then get nuked into oblivion, they’ve had the ability to level Soeul with artillery for fifty-odd years. The fact that they haven’t done so suggests they don’t want to.
I don’t think Pakistan or any other country will ever give a nuke over to a third party. The consequences of using one are too severe to let some outside group decide when and where it would be used. Not to mention for places like Pakistan and N. Korea, a nuke probably represents a non-trivial chunk of their GDP.
I think I’d answer the OP with “no one”. I don’t think anyone will ever use a nuke on an enemy target again. The downsides are too severe, and the upsides too small in just about every conceivable scenario. Pakistan might really want to control Kashmir, but if they launched a nuke against India they’d 1) suffer far worse retaliation, to the point that there probably wouldn’t be a Pakistan anymore and b) Still wouldn’t get to control Kashmir.
Nuclear weapons, Jack. They mean nothing. Everybody’s got them, no-one has the balls to use them. Am I right?
Not a chance; they know that both we and Israel would love the excuse to inflict nuclear genocide on them in retaliation.
I think the best odds for a nuclear strike are from orbit, with the target being population (not farming or production) centers in any countries which don’t bow down to our new alien (possibly robotic) overlords. And, they might not even use nukes; they could just drop rocks on us, or use flying crowbars.
For my reward, I’ll take a “Go Pods!” travel mug.
This is a hopelessly optimistic position which assumes that nuclear arsenals will always be controlled by rational actors who possess accurate information. The reality is that even with only a small number of nuclear powers there have been a number of near exchanges. As the number of nuclear powers grow and the controls over nuclear weapons and nuclear proliferation become less stringent the likelihood of an attack due to accident, security lapse, or intentional attack approaches unity. Nations–even major nations with international ties–cannot be counted on to be rational actors at all times.
Stranger
Well, that’s sort of my point. I don’t think N. Korea is a rational actor, and I doubt they have particularly great information about their enemies either. But they don’t need to, they just need to not be actively suicidal for no reason, and just need to know enough to realize destroying Seoul would mean they’d be wiped off the map without actually accomplishing any of their goals.
Neither of those sets a particularly high bar. The downside is so huge and obvious while the up-side is so negligible that even even most irrational and stupid actors would understand them.
Which isn’t to say I think the risks are nil, and since the potential downside is so huge I certainly think non-polifieration and disarmament efforts should continue. But I think the chance is small, and so my best guess to the OP’s question is “no one”.
Conspiracy theory time! 
Both India and China have huge overpopulation problems. A limited nuclear exchange could solve that, provided they have enough nukes. Perhaps they engineer a flashpoint over Tibet or Bhutan, the nukes start flying, then Russia steps in as a peacemaker and everyone steps back, secretly smiling that there’s half a billion fewer mouths to feed.
Right, 'cause the easist and cheapest way to desl with excess population is to make large tracts of land uninhabitable and create millions of refugees suffering from famine, dystenery, and radiation sickness.
Stranger.
Remind me: how’s Hiroshima doing these days?
If we’re doing conspiracy theories:
The North Korean government, knowing everything is about to fall apart for them, sets off a nuke at one of it’s more remote military bases.
It then launches a full diplomatic and media blitz blaming the US/S.Korea/Japan/South Park and in the resulting confusion sends it’s armed forces South in a final hell-or-glory charge.
Do you have any clue as to what you are blabbering on about? It took four years and the equivalent of tens of billions of dollars to rebuild Hiroshima. Occupants of the city suffered from elevated levels of radiation related illness and a disproportionate incidence of birth defects for years afterward. And this was from a device (“Little Boy”) which had a tiny yield and was not optimized for neutron and high energy x-ray yield unlike modern “city busters”.
China has never had a problem eliminating excess population via the classical and time-tested method of controlled famine and disease. The biggest internal problem that China has today is not the shear volume of population, but the demands that upward mobility and a higher standard of living are making on their infrastructure, energy, and transportation fuel systems.
Stranger
Why would they need this excuse? If they are going to go hog wild in a death charge which nobody–not even their nominal benefactor, the Peoples Republic of China–would support, they don’t really need an excuse.
And it would be readily obvious via isotope ratios where the weapon material was produced. Nearly all American weapon-grade nuclear material was produced at the Savannah River site, and the isotope characteristics of material produced in those reactors are publically available. Ditto for most other nuclear-capable nations; we would easily be able to demonstrate if a weapon were sourced from Russia, China, France, Great Britain, or South Africa, and probably India and Pakistan, too.
Stranger
I’m no expert, but unfortunately yes, I do have some clue. Do you?
So what? Let’s assume 300M casualties and $30B to clean up - we’re not interested in more than modest rebuilding and as you say, the Chinese government aren’t too concerned about their citizens. That’s $100 per person: not a lot, really.
Uh huh. Or they can just reduce the food supply to <800 kcal/person/day and wait for people to start starving to death, or divert the water supply and force people to drink non-potable water, which has been the historical way that China has dealt with excess population. These methods costs virtually nothing, and don’t require some kind of elaborate and obtuse conspiracy between nations which are militarily constantly at each other’s throats.
Stranger
Well you can show the weapon didn’t come from any of those countries* if *you can get samples from the area - good luck with that when the N. Koreans are all *“Go for the eyes, Boo, go for the eyes!” * on the South.
As for an excuse, all it has to do is cause enough confusion and speculation on the international scene to delay or hamper a concerted response long enough to meet their goals.
Plus, if you’re the US and have been wrongly accused of using nukes once in a pre-emptive stike, would you be as ready to use them again to stop the N. Koreans?
No, you don’t need to get samples from the ground. There will be residual material from incomplete fission in the air. It only takes a very small sample to confirm isotope ratios. And no, there isn’t going to be a delay on a response to an invasion or attack by North Korea on South Korea. US Forces Korea stands at close to thirty thousand troops with armor, artillery, and air power. An attack upon US forces in South Korea would provide an immediate response by the US regardless of whether North Korea claims it was attacked first or not.
Stranger