General David Petraeus had just assumed his new role as US Central Command chief in 2009 when he began introducing his staff to a young Harvard University researcher who was writing his biography. The woman, Paula Broadwell, then 37, had never written a book and had almost no journalistic experience. But that wasn’t the only thing about her that made the general’s aides nervous.

Petraeus — already the most acclaimed US military commander in recent decades — had until then been extraordinarily careful in managing his public image, allowing limited access to a handful of journalists, former aides say. Yet, when it came to Broadwell, he seemed eager to throw his own rulebook out the window.