Actually I think the European thing is just an example of a broader phenomenon: democracies never go to war with other democracies.

(I went googling to make sure that was true, and found this: http://users.erols.com/mwhite28/demowar.htm which goes through all the potential counterexamples. So I'll amend my statement to say that democracies hardly ever go to war with other democracies.)

So the idea is really a bit redundant -- two countries probably won't go to war if the people have anything to say about it anyway. Only when you have decisions being made by a dictator on at least one side are disagreements likely to turn into wars; democracies tend to compromise.