Technology did it.
WWI started out like everyone thought wars were. Commanders with feathers in helmets sitting on horses, while rows of soldiers fought it out in sequence. Until someone decided to dig a hole and hide, which lasted until someone invented tanks.
That war never ended, so when it turned into WWII we had even more stuff to use for war - and on a much larger scale. It ended with the atomic bomb, of course, which means it would have ended even earlier if the bomb had been finished sooner.
The world will never see anything like that again. Warring now on a global scale involves drones, soon will involve robot soldiers, and involves "the cyber." Coming soon, The Space. It's entirely possible that a future battle will involve lasers or something ripping apart huge parts of the earth, but that's no different from the comet or whatever that's going to crash into us eventually. We've moved on.