Oh, I pray I never see the day. I just think the casualties throughout the world would be more devastation than probably WWI and WWII combined as well as destroying the trade industry and economy. Though, I will challenge myself and be the first responder to your query.
This is an extremely difficult question and it would depend on which allies teams up. I think most would say United States and Russian Federation and even China, North Korea as well as countries like India and Israel which are very strong in arms would be ranked high on the list. United Kingdom has lots of friends and can gather forces from other countries that can be very persuasive. Don’t underestimate the Persian countries too, which I believe have something hidden away no one knows about.
With all this said and of course some may argue over, I would like to be the Devil’s Advocate and go with those countries that are neutral and there are many. This is why I say this. Most neutral countries are very strong economically as well as strong defenses. These countries will sit back and watch the superpower countries annihilate each other and sadly decrease the population by a number I cannot imagine. Once they are finished destroying each other, there won’t be much capital to rebuild their defenses for they will be bankrupt. All the neutral countries have won by do I dare say the “Sudden Death” rule.
These are just my thoughts whether right or wrong as I pray there will never be such a war involving the world. Like my signature states: “Love Understands All Languages” I truly hope the world will one day understand the love and live in peace!