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.