A country starts a war, fundamentally, when they see a resource (land; oil; people) that they think they can claim without destroying its value in the process. However, all the technologies we've developed since WWII that make us more efficient at mass-killing (chemical/biological weapons et al) are politically-blind; Anthrax will infect the conqueror as much as the conquered. The only real advance we've made in the "things that will kill the enemy but leave you alone to sit in their houses and farm their land" field, in the last 70 years, are drones--and, tactically-speaking, they haven't increased the
difficulty of war much for the side of the invaded, just made waging a war
safer for the invader. So, it's not any
easier to win a war against a nation at technological parity with your own than it was 70 years ago.
On the other hand, over the last 50 years, besides there having been built within every world-leader a strong awareness of the "final solution" (nukes) available to the major powers to put down any sufficient threat, a true global trade economy has also emerged, with strong economic interdependence between the major powers (China holding US debt &c). Every power is now just as much beholden, economically, to each other power, as if they were an export colony; the US going to war with China, for example, would be cutting off one's nose to spite one's face in the same way that Britain choosing, without provocation, to start warring with America in 1775 would have been. In this situation, I don't see much chance of a major power instigating an empire-building war.
Also, unlike in WWI, a country that isn't a major power doesn't have the political weight to instigate a major war--nobody will follow it into battle. Attacking a country in a union with a major power is implicitly attacking that power, and military strategists now treat it that way, instead of thinking they can "just" take the country. Instead, we merely see individual unallied states getting nasty to their unallied neighbors--free radicals bumping into one-another--and having to be calmed down by some third party. I would guess future "wars" all across the globe will look more like the US's dealings in Afghanistan/Iraq than it will like some sort of confrontation between NATO and some new Axis.
Another way to put it, is that national political borders across much of the globe have basically annealed down to a rest-state. The resignation to this fact is what seems to have spurred the EU into existence; if you can't invade your neighbor any more, you may as well cooperate and trade. I see this sort of idea spreading (though perhaps without the shared monetary policy aspect, which doesn't seem to be working out too well) as more regions of the world "settle", leaving basically four or five pretty-similar political bodies in a de-facto world-government union, and individual break-away states here or there which are filled up with "peacekeeping forces" by the one or another of the powers.
I think this logic is pretty common to people who don't have a dystopian view of the future. Do you see some major flaw in it?