An M1A1 Abrams Main Battle Tank from Bravo Company, 185th Armor, 81st Armor Brigade, conducts an area reconnaissance around Balad, Iraq on Sept. 6, 2004. (U.S. Air Force photo by Staff Sgt. Shane A. Cuomo)

This is a guest post by political scientists Elizabeth Saunders and Todd Sechser.

Monday brought the news that the proposed U.S. defense budget would significantly decrease the Army’s manpower.  The downsized Army would be ready to fight a conventional war, but would not be large enough to undertake long-term occupation or stabilization missions like the wars in Iraq and Afghanistan.  It is tempting to see the move as a cost-cutting measure.  But while budget constraints, as well as a new emphasis on China and Iran, are part of the story, the proposed shift in U.S. force structure is also a conscious choice—and, it turns out, a predictable one.  The United States is responding like most other nations that have fought difficult counterinsurgencies abroad.  The question, however, is whether this change will stick.

In one sense, the proposed defense budget seems like an odd step. The United States often struggled to fight counterinsurgencies in Iraq and Afghanistan.  This is not to say that these wars were defeats for the United States in the way that the Vietnam War was.  But they were also not clear victories.  So why then did Chuck Hagel and other Pentagon leaders not respond by doubling down on the military’s capacity to conduct counterinsurgency warfare, as some have been advocating for years?  Why not learn from the wars in Iraq and Afghanistan by investing in counterinsurgency to make sure things go better next time?

Our research points to one answer.  We examined how countries structure their military forces — specifically, the mix of manpower and armored vehicles like tanks.  More “mechanized” armies — those with a mix tilted more toward armored vehicles — are good for fighting conventional wars but less effective in counterinsurgency wars, where more manpower is needed and armored vehicles can get in the way.  We looked at how a variety of factors influence a country’s mechanization rate, including their recent experiences fighting wars.

After countries fought costly counterinsurgencies, we found that they appeared to learn from the experience.  After accounting for other factors, countries that had recently suffered counterinsurgency losses or draws had lower rates of mechanization, on average, than countries that had not — that is, they invested more in manpower, presumably to help them fight the next counterinsurgency.  But here is the key point: we found this relationship only for countries that fought counterinsurgencies on their own soil, where the conflict represented a clear internal threat.

By contrast, outside interveners or occupiers that recently experienced counterinsurgency failures abroad had slightly higher mechanization rates, on average, than countries that had no recent counterinsurgency losses or draws at all.  The Pentagon’s decision to invest less in manpower and presumably increase the military’s mechanization fits this pattern precisely.   (The new U.S. defense budget emphasizes air and naval weapons rather than tanks, but the general point remains that the military seems to be emphasizing weaponry more and manpower less.)  Indeed, our article concluded by observing that “unless less-mechanized adversaries come to occupy a more central place in the hierarchy of threats confronting the United States, we should not necessarily expect the United States to make major, enduring changes to its military force structure in the wake of the occupations in Iraq and Afghanistan.”

Why do countries draw such different lessons from fighting insurgencies at home or abroad?  Countries that fail to defeat insurgents at home know that they may have to fight a counterinsurgency campaign again, and regimes themselves may be at risk.  But countries that undertake counterinsurgencies abroad can afford to react by simply vowing “never again” and ostensibly swearing off such wars.

And this leads to an irony.  The new defense budget makes it seem as though the United States is getting out of the counterinsurgency business and reorienting around potential adversaries like China and Iran.  But history shows again and again that U.S. presidents often call for precisely the types of missions that the defense budget seems to be swearing off.  So it remains unclear whether downsizing the military will really “tie the hands” of leaders who might want to mount a large intervention, even if, as David Edelstein argues, such interventions are a bad idea.  Of course, force structure is not the same as doctrine, and the Army would retain a lot of expertise and hard-won lessons from its last two wars.  But if the president calls, a downsized and restructured Army would have to make big adjustments.

To paraphrase Donald Rumsfeld’s famous pronouncement, don’t be surprised if we end up going to war again with the army we have.