It's not simply democracy that made a difference. The general increase in wealth, and the wide acceptance of the need to redistribute that wealth in order to create stable, prosperous and more or less equitable societies contributed too. And of course, the recent memory of the horrors of the orgy of death that was the beginning of the twentieth century has served as a powerful warning against warfare in those nations most able to keep things in check.

Elections alone don't fix things - we see that over and over again in countries where real resource conflicts exist. 

I also think that the basic numbers of 250,000 deaths or so a year sound low. In the African World War alone (1998-2003, although one can argue it is still going on), about 1 million people died each year. In the Rwandan genocide in 1994, about one million died. Both of those conflicts were undoubtedly international conflicts, if not exactly traditional wars. In Iraq from 2003 to 2008 there have been on average more than 100,000 additional deaths a year by epidemiological estimates. This is in a war of choice, initiated by a democracy. 

The fact that the core of the world order consists of wealthy socialist democracies with a strong pacifist culture formed by the experience of total war, has served to contain international conflict for decades. Where this pacifist core isn't fully committed - like in Africa - we see no change in the prevalence and deadliness of war.  The main question is whether these countries will stick to pacifism while their wealth is redistributed to emerging powers like China and India. 

You'd need to make a better case for the socialism, especially since it was socialist nations responsible for much of the 20th century's massacres.

Capitalism has a heck of a lot to do with the wide distribution of wealth, as well as relative peace between nations where international trade has (somewhat) obviated the need to grab territory.

Every developed country in the West is essentially socialist, providing basic services like healthcare, education and housing to all by redistributing wealth. There are big differences in the degree, but not in the principle. By creating a vast middle "class" where people can see the value of working hard, and the benefit of working within the rules, socialist democracies have reduced the number of people that have nothing to lose and those that have no respect for society's laws. That curtails violence in general and removes a lot of tinder that firebrands can use to rally nations to war. It's no surprise that the least socialist nation is also the most violent and warlike.

endorendil:

Every developed country in the West is essentially socialist, providing basic services like healthcare, education and housing to all by redistributing wealth.

Every developed country in the West is a hybrid. You credit the spread of peace to their socialist tendencies without any proof; well, anyone could just as easily credit the spread of peace to their capitalist tendencies.

It's worth pointing out that Germany was probably the most liberal and socialist society in Europe in 1914. Its government was under the sway of militarists, of course, but that had nothing whatsoever to do with the economic system; the worst militarists were from the most backward and isolated part of the country.

endorendil:

By creating a vast middle "class"

What utter rubbish! Socialism has nothing to do with the middle class; the middle class is and always has been purely the poster child of capitalism. People in the middle class don't need government handouts, they are in part the ones who pay for them.

endorendil:

where people can see the value of working hard, and the benefit of working within the rules, socialist democracies have reduced the number of people that have nothing to lose and those that have no respect for society's laws. That curtails violence in general

Is that why violence is so much lower in subsidized housing developments? (And not just in the US, the same holds true in the UK.)

I think you're missing endorendil's point somewhat Bruce. He doesn't seem to be talking about Socialism with a capital S, and the idea of government handouts, but rather the notions that those involved in the production of wealth should have an equitable share in that wealth, and that government has some responsibility to intervene in order to promote a general societal good. Both those ideas were quite revolutionary in the 19th century, and whilst capitalism certainly transformed the economic model for the better, the idea that more should benefit from the new order can certainly be attributed in part to socialism.

Socialist democracy is indeed a hybrid, Bruce. Unfettered capitalism is what the world pretty much had until socialist ideas gained acceptance in the west and workable hybrid forms were established (from the thirties to the fifties) that created economies that actually worked for the majority of the people. The middle class as we understand it today, was not a concept until the fifties.

As Einhard points out, I'm not talking about big S Socialism. Its idea of a permanent battle between the upper and lower classes was not intended to give rise to large middle class, but to give control to the lower class. That may have looked like a reasonable alternative at the end of the nineteenth century, when it seemed that big C Capitalism could indefinitely keep the small upper classes on top at the expense of the majority, even in democracies. The modern western solution is to diffuse the (still very real) antagonism between low and upper classes by creating a bell curve with most people in the middle and sufficient mobility to keep everyone interested in playing along.

It is important to keep in mind that all these factors - wealth, socialism and democracy - play a role in creating the kind of societies that the west now enjoys. China and India are experiments in recreating our success story on a much, much larger scale (each of these countries have more people than in the EU and US combined, and when we went through this evolution, the populations were smaller and more homogenous still). It should be obvious that this isn't straightforward. In particular, the relative success of China and India shows that democracy isn't necessarily the most important part of creating a largely peaceful and prosperous country. China's middle class has exploded from a small minority to almost two thirds of the population in less than two decades, without even a glint of democracy. Finally, to come back to the article, I expect that China will eventually become more democratic, but I think that that will not necessarily lead to a more stable or a more peaceful country.

TheyCallMeBruce:

It's worth pointing out that Germany was probably the most liberal and socialist society in Europe in 1914. Its government was under the sway of militarists, of course, but that had nothing whatsoever to do with the economic system; the worst militarists were from the most backward and isolated part of the country.

There was never Teddy Roosecelt style trust busting in Germany. Industry was heavily monopolized and cartelized. This Duetschland Inc was making lots of money from the arming of their country. The capitalists hoped that the coming war would force the British and French to cut Germany in on the colonial empire action.