It is interesting how many times in history the development of international trade has led to wars. The story is similar each time. Open your country to international trade. Some important item starts coming to you from abroad at lower prices (of course, otherwise it wouldn’t be sold instead of domestically). Often food, but it can be other items as well. According to economic theory, this is supposed to lead to fiercer competition and increased efficiency, but in real life it leads to the bankruptcy of many domestic firms. Your country is dependent on imports, and suddenly it is vital for you to gain control over routes, over political conditions in other countries, etc. You can very easily get into a situation where it is very difficult to avoid war.
The situation in the US and Europe today is really just another part of the series. The free-market answer is constant wars, the Trumpian answer is restoring self-sufficiency.
Economically self-sufficient countries can choose to go to war or not. Import-dependent countries have no choice. That is a stronger factor than whether a country is democratic.
Build national economies, you strengthen peace!