The rapid collapse of Afghan security forces was not unexpected. In fact, it is almost typical of local security forces established with U.S. military assistance. The United States has failed spectacularly in its efforts to build three partner armies—in Vietnam, Iraq, and now Afghanistan. As images poured in from Kabul, there was good reason to recall similar scenes from Saigon in 1975 and Mosul in 2014.

What the U.S. military refers to as “security force assistance,” “partner capacity building,” or “train and equip operations” remains a central pillar of American defense strategy. Beyond Afghanistan and Iraq, the United States spends billions of dollars each year and deploys thousands of personnel to train and assist foreign militaries around the world. Although the purposes of this assistance vary, the main objective is to enhance the ability of partner militaries to shoulder local security burdens, thereby allowing the United States to shift its resources toward more pressing priorities.

The problem, however, is that U.S. partners are often not genuinely interested in building capable fighting forces. As Georgetown University professor Caitlin Talmadge has pointed out, political and military leaders must promote competent officers, enforce command structures, encourage rigorous training, and root out corruption in order to create effective forces. But in weak or failing states—the primary recipients of U.S. security assistance—leaders often prioritize their own political survival over strengthening their militaries. These leaders frequently aim to use their armies as tools of patronage or as weapons against domestic political rivals. While they may gladly accept U.S. military aid, they often fear building professional forces that could potentially threaten their grip on power. As a result, they ignore the advice of U.S. military advisors and pursue policies that keep their armies weak.