The career-ending affair that led to the resignation of CIA Director David Petraeus had its roots in a multi-year friendship, a flattering book and a young writer who was taken with the four-star general's leadership style.

The affair came to light during an FBI investigation of a complaint that author Paula Broadwell, 40, was allegedly sending harassing e-mails to another woman close to Petraeus, a U.S. official said. That probe also uncovered e-mails related to Gen. John Allen, the top U.S. commander in Afghanistan, and put his nomination for NATO commander on hold.

Here's a look at the anatomy of the affair:

Spring 2006

Broadwell meets Petraeus when he speaks at Harvard, where she is a graduate student, according to preface of "All In" -- the book Broadwell wrote about the general's career and his impact on the military. She told the general about her research interests and he agreed to put her in touch with people studying the same issues. "I later discovered that he was famous for this type of mentoring and networking, especially with aspiring soldier-scholars," Broadwell wrote.

January 2007

The Senate confirms Petraeus as the commanding general for U.S. troops in Iraq.

Early 2008

Broadwell begins her Ph.D. thesis on Petraeus' leadership skills. Some of the interviews were done via e-mail. Others were conducted as Broadwell occasionally ran with the physically fit four-star general, including one time with Petraeus and his team along the Potomac River.

July 2008

Gen. John Allen begins his assignment as deputy commander of U.S. Central Command (CENTCOM) in Tampa, Florida, serving under Petraeus.

October 2008

Petraeus is named commander of CENTCOM, based at MacDill Air Force Base near Tampa, where Jill Kelley and her husband attend social events alongside the area's military elite.

February 2009

Petraeus is diagnosed with prostate cancer. Two months of radiation treatment at Walter Reed follow.

Broadwell moves with her husband Scott and two children to Charlotte, North Carolina.

June 2010

Petraeus is tapped to replace Gen. Stanley McChrystal as the top commander in Afghanistan. He is confirmed by the Senate on June 30, and takes command on July 4 of that same year.

Allen is named as temporary CENTCOM commander, replacing Petraeus.

Broadwell decides to turn her research into a book and visits Afghanistan multiple times. "We had a relationship before I went there as far as this dissertation was concerned, so it just took it to another level," Broadwell told CNN's Brooke Baldwin in February 2012.

July 2011

Allen succeeds Petraeus as the top American commander in Afghanistan.

August 31, 2011

Petraeus retires from the U.S. Army and is awarded the Army Distinguished Service Medal.

September 6, 2011