I heard recently that the science that the concept of mutually assured destruction was based upon was a well intentioned lie. Humans tend to overestimate their importance. Some people are more resistant to radiation than others, and while birth defects increase quite a bit, there are still many healthy babies born in irradiated zones. The main radiation depletes very quickly after the blast, and the cone of wiping out all life on earth is based on the nuclear winter, not radiation. I heard this is also a myth, and that while it would result in a much colder climate worldwide still enough solar radiation would come through the debris clouds to grow plants, and it would be possible to run heaters and UV lamps using nuclear reactors.
Besides, the weather patterns would shift all the debris around so there would be clear spots everywhere anyway from time to time, and I heard that there is no way it would take decades for the dust to settle. Also, by the time the dust settles the radioactivity present in it will most likely have subsided below harmful levels. Some people less than 100 yards from ground zero in Hiroshima survived in open, above ground bomb shelters that were only designed to protect from blasts and shrapnel without any injury. Also, a nuclear bomb that is 1000 times more powerful does not destroy 1000 times the area, because the blast emanates spherically, it requires exponentially increasing energy to expand the blast radius.
Not to say that it's not serious, but just saying the extent of the threat of nuclear weapons has probably been over stated for understandable reasons.
War is simply the continuation of politics by other means. So there can be various degrees of success like in any form of negotiations. It depends on the will of the populations, but also on the resources available. No matter how determined Germany was in world War two, there were simply not the resources in terms of population to sustain the war effort. In this sense a war between China and Russia and NATO would likely be much more protracted. Also, the will of the citizens of NATO is not very strong- this generation is very soft and clearly has no stomach for hardship, so it is likely that the NATO powers would descend into civil war when their luxuries started being curtailed.
Russia and China already have more totalitarian systems in place which would be able to exert a greater level of control, but they are also dependent on a lot of infrastructure, like for the internet, that is based in the West and would be swiftly cut off in a conflict situation. There are enough resources in Siberia and Southeast Asia, which would likely fall under Chinese dominion, as well as Africa, which has a growing Chinese presence, to make for a very protracted conflict. It is likely that Africa would be a major battleground of this conflict due to the the prevalence of rare earths there. Most of the materials used in the electronics we are addicted to are mined in China or Africa, and China would be fairly well insulated from any kind of a direct assault due to the ease of deploying defense systems from Russia through the Siberiañ plains, so NATO would likely counter by trying to attack the markets that are the lifeblood of the Chinese economy and limits their access. This would give rise to a revival in American manufacturing as the economic war heated up.
If you want to understand the power, follow the money- while Western backed institutions like the IMF and Bank for International Settlements may be active in the formation of policy in places like Brazil and and India, the new BRICS development bank sponsored by Russia and China will assuredly try to supplant the IMF wherever it can, forming a globalization of resistance to the current dominant schools. Anti American sentiment in South America is also very high due to decades of the US and CIA backing brutal and unpopular dictators there, so you could easily see a bloc of Brazil, Bolivia, Venezuela, and possibly other states forming against American aligned Colombia, Ecuador, and Panama.
You have intelligent and rational players making the decisions here- Putin's popularity is at an all time high because Russians feel the pride of imperial glory returning, however, nobody is going to take any unnecessary risks. It is a simple question of what can be gained. I heard someone once say that in Chinese the word for "crisis" and "opportunity" are the same word.
In Western Asia Russia sees an opportunity to expand its influence through an emerging shiite bloc composed of Iran, Iraq, and Syria, putting pressure on US backed Saudi Arabia by backing Shia in Yemen. This gambit is likely to fail in the long run because of the preponderance of Sunnis in the region, but they can be used simultaneously to secure a presence in the Eastern Mediterranean and to block access for Qatari gas to the large and lucrative European market which is currently the lifeblood of the Russian economy, and would be threatened by a consolidation of NATO power in Western Asia.
In other words, all Russia really has to do in order to win in West Asia is to not lose. This means a stalemate is actually a desirable outcome, and the present offensive in Aleppo is likely more defensive then genuinely aimed at retaking territory.
It would honestly be very hard to estimate who will win this war since there are many factors that cannot be calculated, but I think it is safe to say it will not be like World War 2 where you had a decisive victory in a few short years, but it is rather more likely to be like the wars of the middle ages that spanned generations. I would expect it to last at least three generations, and by the time something resembling "victory" is finally obtained, the political landscape will probably have changed so much that the winner may not even be recognizable to us.
Of course, looking at history is the best way to determine the future, and we can see that fortunes have been fluctuating between West and East, with Roman and Persian empires going at it for centuries. The last few centuries have been characterized by dominance of the inheritors of the western Roman empire, now known as NATO, due to a surge of resources resulting from the conquest of the Americas. This momentum is starting to run out though, and you can see the economic center of gravity of the world shifting to the east, causing the Indian and Pacific Oceans to eclipse the Atlantic as the most important bodies of water. Control of the Atlantic belongs pretty completely to NATO, but as the Atlantic declines in importance expect the battle to heat up in the pacific, where the Russians and Chinese have a much stronger presence.