Stephen Walt says:
In the world of intelligence, extramarital dalliances are dangerous because they create the obvious potential for blackmail. If some foreign intel service found out that a mid-level intelligence analyst or operative was cheating, they might be able to extract sensitive information by threatening to disclose the indiscretion. Obviously, if the director were caught in a similar indiscretion but remained in his post, it would send exactly the wrong message to the rest of the organization. Petraeus clearly understood that, which is why he was correct to submit his resignation.
But couldn't Petraeus have avoided that problem by fessing up to the affair and then going back to work? You can only be blackmailed if the information is secret.
Instead, Petraeus has sent the opposite signal to his staff: he's told them that, in the event you do something in your private life which might compromise security, you should resign. But given most of his staff won't have academic appointments and fat speaking fees to fall back on, how many of them will take that path?
It reminds me of the novel The Marmalade Files, a political thriller penned by journalists Chris Uhlmann and Steve Lewis and set in Canberra. One of the central characters is a transvestite working in a highly secretive part of the Defence Department. If that sounds unlikely, remember that, during the September Lowy Institute event on The Marmalade Files, Uhlmann said many of the characters were based on real people. And as Uhlmann and Lewis point out in the novel, because the transvestite is completely open about her status, there's no question of a security threat.
Petraeus' actions also raise the problem of recruitment for intelligence agencies: what sort of talent are you going to attract if such garden variety moral failings are seen as a cause for resignation?
Photo by Flickr user Jon-Phillip Sheridan.