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.