In this post, I'm going to argue that the trend is somewhat different than many of us realize before offering some thoughts on how to understand the frequency of warfare over time.
First, let's just establish some preliminaries. The frequency of major power war has declined over time, and, as Josh Goldstein has noted, those wars that do occur are killing fewer people than they have in the past. From 1950 to 1989, 180k people a year were killed in war. In the 1990s, that dropped to 100k per year. Since 2000, it has been 50k a year. That's a wonderful thing. And it has to be part of the story.
But it's not at all clear that wars are occurring less frequently.
Below, you will find a simple graph depicting the number of war onsets by year from 1816 to 2007, pooling across interstate wars, intrastate wars, and extrastate wars (data taken from the Correlates of War project). There's a lot of fluctuation there, but to a first approximation, the number of war onsets is increasing (modestly) over time. To be fair, this is not inconsistent with any claims made about the likelihood of major power war. The title of this blog post is deliberately provocative. But if you were under the impression that organized violence was in secular decline, that the international system was growing steadily more peaceful, you could be forgiven, based on the claims some people have made. You would, however, be mistaken.
In a new project with Scott Wolford and Kyle Joyce1, which I'll be discussing at the upcoming meeting of the Peace Science Society, we argue that the systemic distribution of military capabilities has an impact on the likelihood of war onset at every level. That is, we argue that we shouldn't just focus on whether bipolar or unipolar systems are more or less prone to producing wars fought between the major powers, but any war.
We analyze a four-player bargaining model in which A and B negotiate over the division of some good in the shadow of war, knowing that any war that does occur may expand to include C, D, or both. We assume that C is a major power who is sympathetic to B and D a major power aligned with A. We demonstrate that even if A and B know everything there is to know about one another, and even if they face no commitment problems, war may still occur if B has a better estimate of the likelihood that C will intervene on his behalf than does A. The mechanism here is the familiar risk-return tradeoff, but note that there's a unique twist here. In a world where C and D did not exist, A and B would have no reason to fight. And when they fight, it is entirely possible that neither C nor D will intervene. Our model suggests that strictly bilateral wars may be caused by uncertainty over the intentions of the major powers. Thus, it is inappropriate to focus strictly on the incidence of major power war if we are to understand the implications of major power politics.
Our model suggests that the relationship between the actors' capabilities and the likelihood of war is very complex. We're still working out the full details. But some simulations I've run suggest that, to a first approximation, wars between A and B (which may or may not ever involve C and D) are more likely to occur when C and D are roughly equal than when either is more powerful than the other.
Nothing in our model tells us that we should think of A and B as both being formal members of the interstate system. So we assess this by looking at all types of war, not just interstate wars, and certainly not just major power wars. Using the measure of military capabilities I've discussed previously, we demonstrate that the number of war onsets that is expected in any given year, though generally increasing over time, is also responsive to the distribution of capabilities between the two most powerful states in the system. According to one model I've estimated (email me for details if you're interested), we expect approximately 1.3 additional wars to begin in any given year as we move from the most imbalanced conditions that have observed over the past 200 years (when the most powerful state in the system has three times the military capabilities, according to my measure, than the second most powerful state in the system) to conditions of perfect parity (that is, when the two most powerful states in the system are evenly matched in terms of their military capabilities, as I have measured them).
This next graph depicts the expected number of new war onsets in any given year based on a model that accounts for the general trend over time (increasing) as well as the level of parity between the two most powerful states in the system. The parity values are those that are actual observed, while the predicted number of wars is an estimate produced by the statistical model.
1. Disclaimer: while this blog post is based on research the three of us are conducting jointly, I am responsible for the content of this blog post. Especially the objectionable parts. Anything that strikes you as clever was either Kyle's or Scott's doing.