Pretty much any country without a US or Russian military base would be un-hit.
The US, France, the UK, and Russia would probably all be hit directly, and a few US military bases in Turkey, Japan, Italy, Germany, and maybe Bahrain, Qatar, and Afghanistan might be hit. If Russia were very trigger happy, it might lob a nuke or two at ports in Belgium and the Netherlands, maybe EU headquarters in Brussels too, and if the US were really trigger happy, it might nuke a few Russian bases in Kyrgyzstan, Tajikistan, Syria, and the naval base in Crimea. Bottom line though, 90% of the world is not even a target.
It is very hard to say for sure what the climate impacts would be, but they would not be as severe as some answers are suggesting.
Russia might lob a few at some non-nuclear NATO countries, but their arsenal would be spread thin.
In 1990, Russia had the potential to hit the US with 6,139 nukes in a matter of minutes on ICBMs—today that number is under 2,000. Even in 1990, 6,139 nukes was not enough to take out every industrial, population, and military centre in the US, let alone eliminate all of the American nuclear silos and storage facilities.
Russia simply does not have enough nukes anymore to actually hit every target in the US, let alone every target in every country allied with the US.
For the specific FEMA estimate of what a 6,139 nuclear strike on the US would look like, see:
Brian Collins’s answer to Could parts of the USA survive a nuclear war?
A nuclear war, even with thousands of detonations, would not necessarily result in a nuclear winter. Different experts disagree exactly on what would happen, with some arguing that at the rate aerosols are removed from the atmosphere, nuclear winter would be much shorter and less severe than Karl Sagan claimed in the 1970s:
Brian Collins’s answer to Could a nuclear war between the USA and China have worldwide climatic impacts?
Imagine it’s 2019 and someone told you that a worldwide pandemic was going to start soon… A Coronavirus would spread around the world, overwhelming hospitals and causing quarantines, lockdowns, and millions of deaths
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Here are the worst-case scenario climate models taken from my answer above first for summer, then winter (from Robock (2007)):
Would that be worse than the Ice Age? Yes, but it would only be that bad for one year, and as you can see, many coastal regions around the world would experience only a 2.5 degree drop. Inland areas in Russia, China, and all of Mongolia would be hit the worst.
No one believes the 1980s model by Carl Sagan and his cohort about a permanent nuclear winter, even nuclear winter proponents have moderated their climate models.
In Turco, Toon, Ackerman, Pollack, & Sagan (1990) they claimed that 100 oil fields being lit on fire would cause a global nuclear winter.
Less than one year later retreating Iraqi forces lit 800 Kuwaiti oil fields on fire in revenge:
Temperatures dropped an average of 4° C (7.2° F) over Iraq, Kuwait, Iran, and parts of Saudi Arabia, but the particulate failed to reach the stratosphere and was removed from the atmosphere much quicker than Sagan’s team predicted.
At first, they claimed that they genuinely thought ground-level fires would send large amounts of particulate into the stratosphere, among other flaws in their model.
Later, Sagan claimed in interviews that some of their numbers were for arms control. Basically admitting that they did bogus science showing the world would end if the US and USSR nuked each other, to make it less likely for them to nuke each other and convince the international community to reduce arms.
As for fallout, fallout from airburst nukes would begin dissipating within hours. There would be high fallout areas, but they would be limited to mostly areas in the countries nuked directly, and would be mostly gone within a few months.
Groundburst detonations would leave more fallout in concentrated areas on the ground where the nukes detonated, and would last decades. We can anticipate that all of the groundburst attacks would be on military bases and silos, penetrating hardened targets. Attacks on civillians and industrial areas will be airburst, as that allows the explosion to expand over a much larger area.
That is not to say that un-hit countries would be unaffected. There would probably be food shortages worldwide as two of the world’s biggest food exporters now hit each other directly with nuclear weapons.
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Experts predict that an EMP strike that wipes out electricity across the nation would ultimately lead to the demise of up to 90% of the population.
However, this figure begs an important question: if we were able to live thousands of years without even the concept of electricity, why would we suddenly all die without it?
References:
Robock, A., Oman, L., Stenchikov, G. L., Toon, O. B., Bardeen, C., & Turco, R. P. (2007). Climatic consequences of regional nuclear conflicts. Atmospheric Chemistry and Physics, 7(8), 2003-2012.
Turco, R. P., Toon, O. B., Ackerman, T. P., Pollack, J. B., & Sagan, C. (1990). Climate and smoke: An appraisal of nuclear winter. Science, 247(4939), 166-176.
Edit:
No, I am not saying nuclear war is a good idea, or is advisable, or that the US actually would do this over Syria of all the crazy ideas. Nor am I the OP of this question, as some answers seem to assume, and no I am not lying about being American—I have been on Quora since before I moved to Australia, and I have met some Quorans in real life.
Nor am I saying some countries would be unaffected. Note how I used the word ‘un-hit’ in my introduction.
I have gotten about two dozen comments saying I am wrong because I did not account for fallout. I linked my other answer with the FEMA’s 1990 estimates on fallout from 6,139 nuclear weapons landing on the US.
Here are screenshots from the report in my answer. About half the of the US had a medium to very high fallout risk, and about half had a low fallout risk:
So basically, half of the US population would need to take shelter for a few days to 2 weeks, or risk radiation exposure in a 6,000 warhead strike.
In the high fallout areas, where deadly radiation would be hitting people for more than a week, this is what fatality rates would look like:
This is the US—one of the countries which would obviously be hit directly. Most countries would just have higher than average cancer rates (maybe .3% or .5% of the population would get cancer instead of .1%).
Canada would have some high fallout zones in Ontario, and they would likely take casualties from that. Mexico would have some high fallout zones in Sonora and Baja California Norte.
The 1990 FEMA Report analysed the fallout for every state, and published the data for every state. Hawaii, for example, had no areas of high or medium fallout. Washington looked like this assuming strikes here:
The fallout looked like this:
Here’s California:
If not even everyone in the US has to worry about fallout from a 6,139 nuclear strike (again, Russia has 1/3 of that arsenal today with significantly weaker nukes per the START treaty which limits the throw-weight to 1 megaton), then people on other continents surely don’t. At least not in an ‘everyone is doomed’ sort of way, but maybe worry about it in an ‘I have a 100 times higher chance of cancer so I need to get checked up more often’ way.
The concerns about food shortages are much more worrying than fallout. The US and Russia are two of the biggest exporters of foodstuffs, and collectively feed hundreds of millions of people.
For further reading I also recommend:
Greene, J. C., & Strom, D. J. (1988) book Would the insects inherit the earth?”: and other subjects of concern to those who worry about nuclear war.
It is a book written in response to things like On the Beach (an Australian novel in which the entire world died of radiation poisoning, spreading out of the Northern Hemisphere slowly into the southern), which discusses the benefits of civil defence, studies on how quickly animals and plants recovered from nuclear tests, etc.
The strange title is a play on the popular belief that cockroaches would be the only life left if a nuclear war happened.
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