Let's not forget that this era of cooperation and peace in Europe has only been around for about 60 years, virtually nothing in the context of human history. It may seem implausible to us now that the current world order should ever be upturned, just as it seemed utterly implausible to the average citizen of Rome or the Assyrian empire that their world orders would eventually come to and end. I agree that the nation-state paradigm will probably eventually run its course and without nations the question is moot, but war in general I'm inclined to believe will be around as long as we are.