You guys are missing another key reason that no major war has happened since WWII, the fact that one side of any perspective big war would be the overwhelming favorite to win, whichever the US is on. In order to get peace in
Europe you would need to get one side to be so obviously dominant that the other side will not push the issue to war. In that vein I would say that either somehow the three emperor's alliance alive or at least have Germany choose Russia over Austria and even France would be unwilling to push for a land war in Europe.
Are you sure about that? The Red Army in 1945 was the most formidable ground force in the world, and the only way the US had any superiority in terms of all out war was its monopoly on nuclear weapons. Without them, a European war between east and west is, as Wellington would say, 'a near-run thing.' I'm not ignoring the West's lead in technology in many areas, but with Soviet manpower and good quality armour and artillery, the US is definitely not the overwhelming favourite. Of course, it only took another four years for the Soviets to develop their own atom bomb. And Korea was as large a war as it got after WW2, and the US, whose forces made up about 90% of the UN troops, was unable to hold on to North Korea when China intervened.
Equally, you don't need one dominant side, as the Cold War showed us. I would argue that that makes war more likely, in early 20th century Europe at least, as the larger power tries to swallow up its neighbours. A few may join it, but the rest will probably form an alliance and fight back. Once this has happened, you no longer have one dominant power - you have two. One is a single nation, the other is an alliance of smaller ones, but the effect is the same.
I agree more that long term peace would be very likely if several Great Powers allied together, such as the ones you mentioned, Germany and Russia for example, but if France, Britain, Italy, the Ottomans and Austria all work together against them, it's evenly matched again. Also, in times of peace, it's hard to get alliances to hold together if there is no obvious benefit, so there may not be 'one side' which is dominant, but instead a loose alliance which has individually powerful members which could break away at any moment.