Christy Bowe / Corbis General David Petraeus attends his confirmation hearing for CIA Director at the Hart Senate Office Building in Washington D.C., June 23, 2011

General David Petraeus announced today that he has resigned as head of the CIA, citing an extramarital affair.

The idea of a senior figure in Washington resigning because of infidelity seems quaint in the post-Clinton era of David Vitter and other still-active public servants who’ve put scandals behind them. Attention will quickly turn to Petraeus’ struggles to establish himself at the agency, the traditional ill will between the CIA and the uniformed military and, of course, Petraeus’ handling of the Benghazi attack.

But the real reason Petraeus was done in by something so easily shrugged off by others in Washington is much simpler than that. In the world of spies, there is a short list of weaknesses you can exploit in your opponents; they include ideology, greed, indebtedness and infidelity. People have spent centuries figuring out how to get people to commit treason. Arguing that the other side is better for humanity, offering to help someone who’s deeply in debt or paying hard cash to the greedy are classic methods. But few are more effective than blackmail, and even in the post-Lewinsky era in Washington, infidelity leaves officials vulnerable.

You can’t lead an organization whose case officers must be impervious to blackmail if you’re vulnerable to it yourself.

Here’s Petraeus’ statement: