At this point wars between powerful nations are almost impossible. Proxy wars are the current fashion, and those tend to be controlled in scale by the puppetmasters behind it. I'm more scared about a great civil war in a powerful nuclear nation. Civil wars are a savage affair where no rules apply.
This book was hugely influential in the years just before WW1 https://en.wikipedia.org/wiki/The_Great_Illusion Premise: the large nations of the world are far to intertwined by commerce and culture. Large scale war would be suicidal and the leadership of these countries would never do something so foolish.
I forget whether it was in medieval europe or in feudal japan ( or maybe both ) where someone wrote that the gun was so horrible and easy to use that it would make war unthinkable. The thinking was that the art and skill of the warrior and nobleness of battle was undone by the gun. Years of training to develop martial skills by the brave knight or samurai were no match for a cowardly peasant with a gun. People forget that the gun was viewed as a coward's tool when it was introduced. A real man would never use such a weapon. Or so they predicted. Boy were they wrong.