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Few imagined that such a dazzling career would have so tawdry and so sudden a collapse.

David H. Petraeus's "Rules for Living" appeared on The Daily Beast Web site early last week, posted by his biographer, a fellow West Point graduate 20 years his junior named Paula Broadwell. The fifth rule, beneath his familiar portrait in full military regalia, began: "We all make mistakes. The key is to recognize them and admit them."

Mr. Petraeus took his own advice Friday and resigned as director of the C.I.A. after admitting to an extramarital affair; officials identified the woman in question as Ms. Broadwell. The full back story is not yet clear, though his affair came to light after F.B.I. agents conducting a criminal investigation into possible security breaches examined his e-mails. The decision to step down was his.

Mr. Petraeus, a slender fitness fanatic, is known as a brainy ascetic. He and his wife, Holly, whose father was the superintendent of the U.S. Military Academy at West Point when Mr. Petraeus graduated in 1974, and their two grown children had long been viewed by military families as an inspiration, a model for making a marriage work despite the separation and hardship of long deployments overseas.

After he began the C.I.A. job in September 2011, the couple settled into a house in the Virginia suburbs and began the closest thing to a normal life together that they had had in years, even if the basement he had designated for a home gym was commandeered for secure C.I.A. communications gear.

After years in war zones, Mr. Petraeus told friends, he was amazed to eat dinner most nights with his wife and to discover weekends again. He told friends that on the day his daughter was married last month, he went for a bike ride of 34 miles, or 54 kilometers.

"It's a personal tragedy, of course, but it's also a tragedy for the country," said Bruce Riedel, a C.I.A. veteran and a presidential adviser.

Like many others in jaundiced Washington, Mr. Riedel wondered whether the affair really required Mr. Petraeus, who turned 60 on Wednesday, to step down and leave the agency leaderless. But under the military law that governed his 37-year army career, adultery is a crime when it may "bring discredit upon the armed forces." And a secret affair can make an intelligence officer vulnerable to blackmail.

The C.I.A. director, Mr. Riedel said, probably felt he had no choice. "I think Dave Petraeus grew up with a code that's very demanding about duty and honor," he said. "He violated the code."

He was the pre-eminent military officer of his generation, a soldier-scholar blazing with ambition and intellect, completing his meteoric rise as a commander in the Iraq and Afghanistan wars. Worshipful congressional committees lauded him as a miracle worker for helping turn the war in Iraq around, applying a counterinsurgency strategy he had helped devise and that was widely viewed for a time as the future of warfare. Then, dispatched to Afghanistan to replace Gen. Stanley A. McChrystal, who had been fired by President Barack Obama, he sought to apply the doctrine he had championed, while also applying an aggressive counterterrorism strategy.

He was fiercely competitive and carefully protective of his reputation. Asked to throw out the first pitch at the 2008 World