UNITED NATIONS -- Secretary-General Antonio Guterres warned in an emergency meeting of the U.N. Security Council that the highly volatile situation in the region risks "escalation, fragmentation and division as far as the eye can see, with profound regional and global ramifications."
"The Cold War is back -- with a vengeance but with a difference," Gutteres warned. And he said the safeguards and mechanisms that managed the risk of escalation between the U.S. and the Soviet Union in the past "no longer seem to be present."
In our wildest dreams we would not expect that questions like the one below will be AGAIN treated seriously.
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:
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:
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 though 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.
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.