Decline in trade and protectionist policies cause great power war.

By analyzing thousands of country pairs over several decades, many researchers have found that increasing trade between two countries lowers the risk of war between them. They have also found that countries that are more dependent on international trade have fewer conflicts than self‐​reliant ones, which provides us all with a security interest in other countries’ global integration as well.

It’s not just trade though; it’s free trade. A series of statistical analyses by the political scientist Patrick McDonald show that the level of free trade between two countries has a larger effect on peace. Countries that engage in free trade are less likely to attack or be attacked than protectionist countries. Free trade removes the barriers and privileges that enhance the domestic power of groups that generally support authoritarian leaders and an aggressive foreign policy.

It has long been known that democracies are more peaceful. According to McDonald’s study, the risk of a war between two countries is reduced by almost 30 percent if they move from little democracy to the highest level of democracy. But in fact, liberal, capitalist peace is even more powerful than democratic peace. If two countries move to the highest level of free trade, the risk of war is reduced by as much as 70 percent. Forget about the golden arches, this is the real McDonald’s theory of peace.

In an era of decoupling and trade wars, there are other authoritarian powers, like China, worth worrying about. It’s not just current trade relations that inform decisions over war and peace but also expectations of future relations. The historical pattern is that countries that expect the international order will stay open mostly prefer to be at peace with it, to reap the rewards, but when they feel that it is closing time, they start fearing the loss of access to resources and markets. Some even go to war to secure them.

If authoritarians lose all commercial, cultural, and personal relationships with the outside world, they have nothing left to lose. They are suddenly free to act according to their own character. And that is scary.

If goods cross borders, it might not always stop soldiers from doing so. But if goods suddenly stop moving across borders, history suggests that soldiers won’t be far behind.