WASHINGTON — Shortly before David Petraeus took charge at the CIA in September 2011, he stood on a sunny parade ground in his Army dress uniform, his wife at his side, and enjoyed a military retirement ceremony unlike any in recent memory.

A band played patriotic marches, a formation of soldiers crisply saluted, and the nation’s top commanders praised him as the greatest officer of his generation for averting a U.S. defeat in Iraq and rescuing the war in Afghanistan.

“You now stand among the giants not just in our time but of all time, joining the likes of Grant and Pershing and Marshall and Eisenhower as one of the great battle captains of American history,” proclaimed Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, as hundreds cheered and Petraeus beamed.

Now, a little more than 14 months later, Petraeus’ boundless career and sterling reputation are in tatters following his resignation as spy chief after admitting adultery with his biographer. What happened to the squeaky-clean leader who seemed to embody all that the Army and the nation glorify — heroism, honor and victory?

The answer, former aides and officers who served under him say, is the 60-year-old Petraeus had changed from the vigilant officer who had rocketed up the ranks. Once at the top, he had grown accustomed to the adulation. He grew fixated with his carefully burnished image, and grew more isolated from those who might counsel caution.

After moving to the CIA, he decorated his office with military medals and souvenirs from his combat tours, but had to adjust to life out of the limelight. He lived in a quiet bedroom suburb of Washington after years in war zones during which he saw his wife mostly on brief leaves or when he was called back to testify before Congress. He arrived at the CIA without the aides who had served him for years. In their place were spies and civilians who prize independence, not military deference.

“He was taken out of his comfort zone,” said Steve Boylan, a retired colonel and ex-Petraeus aide. “He spent 37 years in uniform and all of a sudden the routine you had is gone, the people you relied on as close advisers are gone. I think he was feeling a little vulnerable.”

Others saw a simpler explanation for his extramarital affair with Paula Broadwell, a West Point graduate and Army reserve officer.

“I don’t underestimate the power of human beings to be attracted to each other when they have a lot in common,” said a former senior CIA official who counts Petraeus as a friend. “He was her mentor; she was his admirer. The only thing extraordinary here is that he is so famous.”