The CIA director's path to resignation began with some email messages.

Reuters/Joshua Roberts

While David Petraeus was serving as a four-star general in the U.S. Army, he began exchanging emails with the woman who would eventually write his biography. After Petraeus retired from military service and accepted a new post -- director of the Central Intelligence Agency -- those email exchanges, it's believed, turned into something else: an affair.

The relationship might have concluded as so many others do -- temporarily clandestine, permanently concealed -- were it not for the technology that helped to ignite it in the first place. The FBI, the Wall Street Journal reports, began to suspect that Petraeus's personal Gmail account had been hacked -- so it launched an inquiry to determine whether someone else had been accessing his email messages. In the process, the agency discovered what Google's servers had known all along: that the head of the CIA, the keeper of the country's secrets, had been keeping a secret of his own.

So how could a (potentially) hacked email account lead to the resignation of one of the most powerful men in the nation? Why, if the account in question is a personal one, does it matter?