I am not sure there is an inherent value in democracy that maintains peace. Perhaps the added utility of choice might lead to that, but I am not sure. I can say with confidence, however, that peaceful transition of power is not limited to democracies nor is democracy completely resistant from violent takeover. No system of government endorses violent takeover (or plans it), but they usually happen when someone wants power, the people starve, etc. Someone can violently takeover a democratically controlled government too.
All that aside, I believe that if you are in this to deter war and promote peace, the policy position to take is promoting free trade (not necessarily an inherent democratic value despite a few big historical correlations). Because if there is one thing that most people accept it is that money has a lot of power in the world and when the costs of war are so great that it could have severe ramifications for a country or more yet the people and their livelihoods, the benefits of war quickly get drowned in the benefits of peace.