If it is confirmed that it was Russia government that nuked UK, then Russia would be hit by nukes – it no longers matter if Russian militants physically entered Ukraine or not .
The UK has 120 nukes which Russia could only hit in submarines that were docked in British ports.Should Russia nuked U.K with or without its soldiers entering UK, the UK would launch its entire arsenal at Russia, and the US and France would join in.
Russia would have to spread its 1,800–2,000 strategically deployed nukes (assuming it used them all) over the three countries, whereas the US (1,411 nukes), France (290 nukes), and UK (120 nukes) would use their combined arsenal of 1,821 strategically deployed nuclear weapons on Russia. The UK would do this whether or not any other NATO country participated.
Russia would probably nuke a few strategic chokepoints with significant US Naval assets around the world as well, such as Naples Italy, Bahrain, and Okinawa.The US might drop anywhere from a couple hundred more nukes on Russia, to a few thousand from its aircraft, depending on how many survived the initial Russian strike, and depending on just how bloodthirsty the American people were after losing probably over 50,000,000 citizens, and many cultural landmarks.
In addition to its 1,411 strategically deployed nukes, the US has 3,069 nukes, mostly in mid-range missiles and also in bombs, ready to be dropped from aircraft.
Russia also has an additional estimated 2,000–3,000 nukes which, like the US, are built to be dropped from aircraft. Realistically, I don’t think it is plausible for Russian nuclear bombers to penetrate NORAD, even after a nuclear strike, especially since Russia has 1/15th the number of airports as the US.
The US has 15,095 airports, while Russia has probably under 2,000 strategically deployed nuke.Even after a full nuclear strike, the US Air Force and Naval Aviation could use the surviving civilian airports and roads to launch strikes on Russia, whereas it is physically possible for the US to nuke every airport in Russia, leaving the country with absolutely air infrastructure.
Many US bombers and transport aircraft are designed to be able to take off on gravel runways, and many US aircraft can land on targets as short as carriers.
At this point, NATO countries in Europe would probably be sending in everything they have, especially Eastern European NATO members, and probably Turkey who would want to annex little pieces of Russia.
Plausibly, the US would avoid nuking many of the border areas to avoid collateral damage on its allies.In the end, probably 200,000,000–300,000,000 people would be dead from nuclear explosions in North America and Europe, and over the next year millions of people would starve as the two biggest breadbaskets just threw everything they had at each other.
Unlike Russia, the US is fortunate in not being bordered by countries that are allied with the opposite side. The US would possibly never recover fully from losing 1/4–1/3 of its population, and a lot of its arable farmland for years, but it won’t be invaded.
There is a debate on how climate would be effected. Most experts now beleive this would last for a few years. This is the worst-possible scenario from Robock et al., (2007).
Again, Russia would be hit the worst, but Europe would be a lot colder, as would Canada and the US.For further reading on different country’s nuclear capabilities see:
Nuclear weapons of the United States
Nuclear weapons and the United Kingdom
Russia and weapons of mass destruction
