It could go nuclear.
If Russia/China/USA are involved each one is too big to be taken down by the other with conventional weapons and if it went nuclear the "winner" would be mortally damaged itself.
But let's say India and Pakistan square up to each other (Political/religious divide - Hindu and Muslim). Let's say due to the sheer size India is winning the conventional war. I could see elements in Pakistan "taking them down with us" and nuking major Indian cities.
Fortunately the bigger boy (India) seems to be the more peaceful one!
So I can see a continent being laid to waste but not the whole globe.
If the middle east - as in Israel and the Muslim nations went nuclear then any nuclear action would be in that region even if it came from any of the three major nuclear powers.
And the UK? I'm glad the UK has it's little fleet of nuclear subs out and about - it makes me feel safer as it did throughout the cold war. (even though Canada, Australia, Germany, Spain, Italy, Japan, Brazil etc etc - even South Korea, get by quite happily without) and I believe that in hindsight the UK could've relied on the USA to be "The West's" nuclear deterrent.