Or at least, a fine illustration of the gulf between the way you and I view the world (and through us, the political philosophies we archetypically represent).

There's no denying that the 20th Century was unlike any century that went before it. On the surface it was characterized by, among many novel things, the scale and ferocity of its wars. But behind that simple fact is the interesting nature of the outcomes of those wars: The aggressors always lost.

War stopped working in the 20th Century.

Sure, we had to fight with everything we had to prevail when we were attacked. But that illustrates a point I made earlier: We won on defense, and failed on offense. The two positions are not symmetrical. The aggressors have always been motivated by abstractions such as ideology or notions such as the domino theory (which we've tried to apply twice now, most recently in Iraq, and failed both times), or cold-blooded calculations of gain. But for the defenders the issues have always crystallized down to life or death--compared to the infinity of destruction, the cost term in the equation is effectively zero. Aggressors could never realize infinite gains, and so the cost term for them was always a significant consideration.

I don't want to waste a lot of time making a detour into "wars of liberation". A true WoL is an internal matter, rebellion or civil war. When third parties have used it to justify intruding and taking sides, it has always been used merely as a propaganda device. The third party was always essentially an aggressor pursuing its own interests, and never went to war out of pure altruism.

In thinking about why war stopped being an effect way of pursuing national interest, the answer I come up with is another striking feature of the 20th: Ever-increasing interconnectedness and interdependence between once-isolated parts of the world. Advances in communications and transportation led to economic interdependence, etc., multiplied by dramatic population increase.

The growth of I&I has an interesting trajectory in the 20th. At the dawn of the century the world saw how a network of interlocking treaties magnified the assassination of Archduke Ferdinand out of all proportion and led to a general war. But rather than conclude that treaties and international relationships were the problem, statesmen had the wisdom to try to perfect those relationships, starting with the League of Nations. The first attempt went poorly, but after the second World War, League of Nations 2.0, a.k.a. the United Nations was established and has done a better job, albeit one that's still far from perfect. It would be a terrible mistake to conclude that international organizations are hopeless because the UN is imperfect; rather, history shows that the wise path is to continue to try to perfect them.

A related thread of interdepence and interconnectedness reducing the prospect of war was illustrated vividly by Mutual Assured Destruction. The US and the USSR handcuffed themselves together in a vow that both would suffer the same fate if one dared start a nuclear war. I wouldn't recommend that sort of thing as a general practice, but as an adhoc solution to the unprecedented introduction of nuclear weapons into the mix, it wasn't a bad idea.

Nuclear weapons and ICBMs created a general form of I&I, since they closely coupled any two potential adversaries and eliminated distance as a buffer that might embolden one to attack.

All in all, the trend in the 20th Century was to make war ever less profitable for the aggressor, and I can't imagine any sane person thinking that that was a bad thing.

I see the world changing for the better, while your focus, Tom, on the eternal reasons why wars start suggests that you see the world as static and unchanging. I think the facts are on my side in this one.

Give peace a chance!