The overall claim is that freer mobility of people who wish to move for peaceful purposes (such as business, pleasure, family reunion) reduces the likelihood and severity of conflicts between nations. If this is true, it would show that the alien invasion metaphor for open borders gets things exactly backwards.

There are two empirical claims:

• Countries which are strongly connected through migration (e.g., a significant portion of people from one country currently reside in the other) are less likely to go to war. Further, if they do go to war, the wars are likely to be shorter and less bloody. Moreover, the relationship is at least partly causal from the migration connection to peace.
• Countries which are generally open to migration are less likely to go to war, even with countries that do not send them significant numbers of migrants. The relationship may be partly causal.

Unfortunately, there is (to our knowledge) very little empirical research that looks into these questions. Thus, the above hypotheses are currently just hypotheses.

However, there are strong reasons that these hypotheses are prima facie plausible:

• The analogy between immigration and trade, plus the fact that there is strong evidence suggesting that the analogous claims for trade are true: countries that trade are unlikely to war with each other, and countries that are generally open to trade are less likely to go to war, even with countries that they don’t trade much with. For a primer, see The Diffusion of Prosperity and Peace by Globalization by Erich Weede for the Independent Institute.
• There are many specific historical anecdotes that suggest that a country with large number of immigrants from another country faces stronger constraints in going to war with that other country.

Blog posts:

• Deepening the Peace by Nathan Smith on the Open Borders blog.