Who makes war on whom?

Romain Wacziarg from UCLA was at UPF today, talking about his latest paper on war and relatedness. He is using the same data that he used in a paper on the diffusion of development (also with Enrico Spolaore, published in the QJE). Its based on genetic similarity analysis - an examination of the extent to which genetic code of humans shares genes that are not immediately useful for (reproductive) success. In their QJE paper, they argued that genetically similar populations are more likely to share the same level of riches. In their latest, they find that they also make more war on each other. Romain's interpretation is that cousins fight each other because they care about the same issues. The result is very intriguing, it survives controlling for distance, and it's robust to a lot of alternative specifications, such as excluding those with shared borders, etc., but I remain puzzled... do we want to think of Canada, NZ and Australia making a decision to fight Germany in World War I and II? Or is it that they are more or less compelled to fight, since Britain is fighting? In other words, what's the identifying variation (countries that are similar genetically, but not close) once you look at units of analysis that are not right next to each other?