In war, it’s not just your allies that matter—their allies matter too. And so do theirs.

Friends of friends of friends are basically friends

The friend of my friend is probably not my enemy. At least, that’s the finding from Ohio State political scientist Skyler Cranmer, together with co-authors Caitlin Clary, Aisha Bradshaw, and Weihua Li. Nations with formal alliances tend to remain at peace, naturally, but according to the study published last week, it’s not just that the allies themselves who play nice.

“We looked at how a country’s alliance portfolio effects not just allies, but also the allies of its allies, and the allies of its allies allies,” says Cranmer. But they didn’t start with the notion that alliances would hold up with so many degrees of separation.

“What we were interested in was the idea that peacemaking ability of alliances is related to direct alliances,” says Cranmer. “We were thinking that this extends outward, to the allies of allies, because if two allies have something in common then an indirect ally probably has something in common with another state, if not quite a lot, given that the bar for establishing a defensive alliance is relatively high. One of the things we were looking for was to figure out what that horizon of that peacemaking ability was, and in our case it turned about to be three degrees.”

This is less about any individual nation’s particular skill at diplomacy, and more just a version of network effects. It’s pretty similar to the kind of network that allows Facebook to find people you might want to be internet friends with. The researchers treat each nation as a node, with the alliance being the link between nodes. Nations that are linked (not to each other, but both to the same intermediate node) are less likely to go to war. As one real-life example, the researchers cite Turkey and Iran during the middle of the Cold War.

From the study:

In 1965, which is the starting point of the data sets used in the study, both Iran and Turkey were allied to the United States, though not directly to each other. And then in 1979, there was a rather big change in Iran—the Iranian revolution drove the pro-U.S. Shah from power and replaced him with the Islamic Republic. So what happened after they lost this mutual ally?

From 1980 until 2000 (which is the most recent data included in the study), five deadly military disputes—none of which hit the 1000 battle death casualty threshold to count as a war, but all of which were violent—broke out between the two countries.

So why do allies of allies make nice? Cranmer says he and his colleagues can’t really be certain, but they’ve started to speculate.

“What we think it probably is, is these communities of allied states are reinforcing norms and reinforcing the status quo so strongly that you don’t see that drop-off effect,” he says. You might not have enough in common with Deb to be friends with her on Facebook, but if you’re both friends with Marco, the fact that you both put up with his nonsense probably means you could find a few things to connect on. And you probably wouldn’t go to war with one another. “But once you get outside of these communities and neighbor communities, that’s no longer there,” Cranmer says—eventually, the benefit of an indirect connection degrades to uselessness. Why is three degrees of separation the lucky number? He and his colleagues hope to explore that in future research.

There are a few examples of states that sit entirely outside this network: Switzerland, which so famously makes no alliances, and North Korea, which is an international pariah. But most states are players in the international community to at least some degree, according to Cranmer, and so they’re connected to an alliance network, a complex circle of friends-of-friends-of-friends. That’s useful information for policy makers, who can consider an alliance not just on the merits between the two countries sitting at the table, but also on how those alliances can help to indirectly cement global peace.

“The gist is that alliances matter more than we thought they did,” says Cranmer. “There’s been a long history of political science and policy understanding that countries that are directly aligned are at lower risk of conflict. But no one has ever really thought before about how that effect ripples out into the network.”