ONCE AMERICA announced that it would not save its client state, things unravelled quickly. As the enemy seized province after province, government soldiers shed their uniforms and ran. On paper the army had hundreds of thousands of well-equipped fighters. In reality its few loyal commanders had to buy ammunition from crooked supply officers and pay in cash for artillery support. The special forces fought well, but regular troops were often commanded by politicians’ incompetent relatives. Soldiers went unpaid as officials pilfered military budgets. Citizens stayed loyal to their families and clans, not to a corrupt government that was as likely to shake them down as to help them. The state was a Potemkin village constructed to please its American sponsors. When they left, it fell.

So it went in South Vietnam in 1975, and again last week in Afghanistan. The similarities between the two collapses are striking. They go beyond intelligence failures, mendacious speeches and abandoned allies. Ultimately, both states fell because they had been hollowed out by corruption, an ancient disease of governance to which America’s nation-building projects are prone. (Think also of Iraq, Kosovo, Bosnia and Haiti.) Political scholars once considered corruption a minor issue, but many now see it as crucial to understanding not just why America’s proxies fail, but how states work in general.