In a statement confirming Petraeus’ departure, Obama made no reference to the reason for the resignation. He said that the retired four-star general “has provided extraordinary service to the United States for decades. By any measure, he was one of the outstanding general officers of his generation, helping our military adapt to new challenges, and leading our men and women in uniform through a remarkable period of service in Iraq and Afghanistan.”

As CIA director, Obama added, Petraeus “has continued to serve with characteristic intellectual rigor, dedication and patriotism. By any measure, through his lifetime of service David Petraeus has made our country safer and stronger.”

Obama tapped the agency’s deputy director, Michael Morrell, a career intelligence officer, as acting director until a replacement for Petraeus is found.

Petraeus wasn’t among the top officials who were expected to resign after Obama won re-election Tuesday to a second four-year term. His departure will force Obama to devote unexpected time and energy to finding a new CIA chief as quickly as possible as the agency grapples with a host of difficult challenges, from the Iran nuclear crisis and the Syrian civil war to the war in Afghanistan, and drone operations against al Qaida and allied Islamists in Pakistan and the Middle East.

“I am completely confident that the CIA will continue to thrive and carry out its essential mission, and I have the utmost confidence in Acting Director Michael Morell and the men and women of the CIA who work every day to keep our nation safe,” Obama said.

In an unusual statement, Senate Intelligence Committee Chairwoman Diane Feinstein, D-Calif., said that she’d wished that Obama had rejected Petraeus’ resignation request, although she added that she understood and respected the president’s decision to accept it.

“At CIA, Director Petraeus gave the agency leadership, stature, prestige and credibility both at home and abroad,” Feinstein said. “On a personal level, I found his command of intelligence issues second to none. He was especially cooperative with Congress.”

Petraeus’ sudden and steep fall was devastating for a generation of young officers he personally mentored.

The former aide said he’d received a stream of emails from other former staff members and veterans of Petraeus’ former unit, the 101st Airborne Division, all dismayed but voicing support. He read excerpts from the messages: “We’d gladly follow him anywhere,” one loyalist wrote; another offered, “I’d punch a fool in the face to defend P4,” a nickname that referred to Petraeus’ four stars.

“My impression is, he’s taking a beating, but he’s still the same guy I’ve known for 25 years,” the former aide said. “What he’s done is horrible, and there’s no excuse. But he feels he’s doing the right thing by resigning.”

Petraeus, from Cornwall-on-Hudson, N.Y., graduated in the top 5 percent of his West Point class in 1974, and married his wife two months later. They have two children.

His Army career centered on assignments in light infantry units, but he also earned graduate degrees at Princeton University. He saw combat for the first time in the 2003 U.S. invasion of Iraq, serving as the commander of the 101st Airborne Division during the V Corps drive on Baghdad.

He oversaw the region around the northern Iraqi city of Mosul, where he developed a campaign that combined combat operations with economic and political reconstruction that curbed the Sunni Muslim insurgency and led to his appointment on his return to the United States to rewrite the Army’s counterinsurgency manual.

In February 2007, President George W. Bush sent Petraeus back to Iraq as the top U.S. commander to oversee a surge of U.S. forces as part of a strategy to crush the growing Sunni insurgency and to prepare the way for the beginning of the U.S. troop withdrawal.

In October 2008, Petraeus took charge of the wars in Iraq and Afghanistan as the commander of U.S. Central Command. Obama then sent him to Afghanistan in June 2010 as the top U.S. commander. He developed the effort to train Afghan security forces and oversaw a surge of 33,000 U.S. troops into the Taliban’s southern heartland before departing in July 2011 and returning to the United States to retire from the Army and take charge of the CIA.

Across party lines, lawmakers praised Petraeus as a patriot who leaves a legacy of four decades of service to the country. They lamented his resignation as a loss for the intelligence community and the nation.

Rep. Peter King, R-N.Y., chairman of the House Committee on Homeland Security, said in a statement that Petraeus is “one of America’s most outstanding and distinguished military leaders and a true American patriot.”