For a lot of very boring reasons having to do with “ethics,” political scientists are not allowed to conduct real experiments in world politics. We can’t tell a head of state, “say, would you mind invading this neighboring country to see if a balancing coalition forms against you?” Our lot in life is hard this way. The best that international-relations scholars can hope for is a “natural experiment.” This is when events change the value of a particularly important variable, and we can then closely observe the effects of that change on world politics. We’re about to experience a natural experiment on the causes of war, and the results may or may not be pretty.
Afghanistan and Iraq have overshadowed a comforting fact—the world has gotten more pacific in recent decades. Click on the Human Security Report and you will find the data to back up this proposition. In modern history, we are in the middle of the longest recorded period without a great-power war. Since the seventies, interstate wars have been on the decline. Civil wars and other forms of violence have been on the wane since the mid-nineties. Although the United States is embroiled in two conflicts, for most of the rest of the world, it has never been more peaceful.
Why is this the case? There are several possible answers, and it has been hard to determine which explanation is the right one. Liberals will point to the rise of the Kantian triad—democracies, international organizations and economic interdependence. All three of these trends have made war much more costly to leaders than it used to be. During the last decade, the intensification of globalization has merely accelerated this trend. Since the days of Norman Angell, liberals have argued that once states realize that they can profit more from peaceful than violent exchange, war will fade as a policy tool.