With rumors of war lately people always bring up WW3 etc..but when I look at the key participants it is basically impossible. I didnt include all countries of course.
Team Trump & Friends
USA European Union UK Ukraine Japan SK Israel Saudi Arabia Australia Canada
Team Putin & Comrades
Russia China Iran Syria NK
Team Putin is seriously at a disadvantage to begin with. If you take away Iran, Syria, NK which I consider "junk players" who would be defeated swiftly that leaves Russia and China. China's strong economic ties to the USA and vice versa would make a war between the two a terrible decision for both countries in the long run. Thats leaves us with just Russia...a country full of poor drunk people run by an aging manlet with outdated military technology.
At the end of the day we will probably see proxy wars in the usual spots (middle east, etc) but the likelihood of a full scale war is essentially zilch.
You're not accounting for the other possibility: The US stars the world war and everyone else fights against us