I expect one in the next 20 years. Reason being there are so many terrorist groups around supported by major countries with alliances with other major countries. Sooner or later one of these groups will nuke a city with 100,000's of casualties that will end up getting their masters nuked in return. I can't see a major US, Chinese, Isriel, India, Pakistan, Japanese, or Russian city getting nuked without nukes flying in return. Europe you can probably nuke and get away with it, same with smaller Asian countries. I included Japan in that list because while they do not officially have nukes sitting in a vault they have all the material, knowledge, and parts to built them ASAP if needed and deploy them.

Both WW1 and WW2 were started by countries fighting over insignificant 3rd parties that snowballed into a mess nobody really wanted.