Osama bin Laden has been killed in an American operation in Pakistan, President Obama announced from the White House on Sunday, calling his death “the most significant achievement to date in our nation’s effort to defeat al-Qaida.”

In a statement delivered from the East Room, Obama said a small team of U.S. personnel attacked a compound Sunday in Pakistan’s Abbottabad Valley, where bin Laden had been hiding since late last summer. After a firefight, Obama said, the U.S. team killed bin Laden and “took custody of his body.”

“We will be relentless in defense of our citizens and our friends and allies,” a somber Obama said in his nine-minute statement. “We will be true to the values that make us who we are. And on nights like this one we can say to families who have lost loved ones to al-Qaida’s terror: Justice has been done.”

The killing of bin Laden – which set off cheers outside the White House gates and lit up the Internet with celebration – will provide a clear moment of victory for Obama at a moment of deep political turmoil overseas that is upending long-standing U.S. policy in much of the Muslim world, particularly the Arab Middle East.

It also comes just two months before Obama is scheduled to begin bringing home some of the 100,000 U.S. troops serving in Afghanistan, a drawdown he promised when he widened the American involvement there at the end of 2009.

Whether bin Laden’s death will have a tangible impact on al-Qaida’s operational capability is unclear, given that, hunkered down in Pakistan’s lawless border region for years, he has served more as the group’s spiritual leader than military commander.

But it will almost certainly help lift support for U.S. involvement in the war, which Obama intends to wind down through 2014, and give the president an irrefutable national security achievement to showcase during his re-election effort. Obama said he has first received intelligence of bin Laden’s possible whereabouts last August, and gave the order Sunday for the operation that ended in his death.

Quoting a senior administration official, The Associated Press reported that Obama gave the final order for U.S. officials to go after bin Laden on Friday. The official added that a small team found their quarry hiding in a large home in an affluent suburb of Islamabad. The raid occurred in the early morning hours Sunday.

Administration officials offered some details of the operation.

Based on statements given by U.S. detainees, intelligence officials have known for years that bin Laden trusted one al-Qaida courier in particular and they believed he might be living with him in hiding. In November, intelligence officials found out where he was living, a huge fortified compound in an affluent suburb of Islamabad. It was surrounded by walls as high as 18 feet high, topped with barbed wire. There were two security gates and no phone or Internet running into the house.

Intelligence officials believed the $1 million home was custom-built to harbor a major terrorist. CIA experts analyzed whether it could be anyone else, but time and again, they decided it was almost certainly bin Laden.

Three adult males were also killed in Sunday’s raid, including one of bin Laden’s sons, whom officials did not name. One of bin Laden’s sons, Hamza, is a senior member of al-Qaida.

Obama spoke with Bush and former President Bill Clinton on Sunday night to inform them of the developments.

“There is no doubt that al-Qaida will continue to pursue attacks against us,” Obama said. “We must and we will remain vigilant at home and abroad. As we do, we must also reaffirm that the United States is not at war with Islam.”

Obama announced bin Laden’s death eight years after Bush declared the end of major combat operations in Iraq, a war spawned in large part by the 9/11 attacks, in front of a “Mission Accomplished” banner on the deck of an air craft carrier. One senior U.S. official, who spoke on condition of anonymity, said the Obama administration is considering burying bin Laden’s body at sea, to prevent the creation of a place of homage to the al Qaida leader.

“We don’t want a bunch of people going to the shrine forever,’ the official said.

That bin Laden was killed – rather than captured – was a victory itself for U.S. officials, who had dreaded the prospect of a long and complicated legal battle if the al-Qaida leader was taken into U.S. custody alive.

With the military brig at Guantanamo Bay no longer being used to house new detainees, and with the country paralyzed by the politics of where and how to try other alleged perpetrators of the Sept. 11 attacks, the logistics of trying bin Laden could have turned the capture into a spectacle. Now, while bin Laden may become a martyr to his supporters, it will be as an invisible hero.

“Every day he was alive was a symbolic victory,” said Dan Byman, director of research at the Saban Center for Middle East policy at Brookings institution and professional staff member on the 9/11 commission. “This is a man we have hunted with different degrees of intensity for more than ten years. … His successful defiance was damaging to the United States.”

Bin Laden, the 54-year-old son of a billionaire Saudi Arabian contractor, was wanted by the United States not only for the Sept. 11, 2001 hijackings but also for al-Qaida’s bombings of U.S. embassies in Tanzania and Kenya in 1998, which killed 224 civilians and wounded more than 5,000 people. The U.S. government had offered a $25 million reward for information leading to his capture or death.

He was one of a handful of Islamic radicals who founded al-Qaida – which means “the base” in Arabic – in 1988 to coordinate the efforts of various groups fighting the Soviet Army in Afghanistan. After the Soviets withdrew from Afghanistan, al-Qaida eventually shifted its effort to target another superpower – the United States.

In what appeared at the time as a quixotic campaign, al-Qaida embraced a terrorist agenda to pressure Washington to withdraw U.S. troops from Saudi Arabia and cease its support of its allies in the Arab world. In 1996, bin Laden and al-Qaida issued a written declaration of war against the United States.

There had been no definitive sightings reported of bin Laden since December 2001, when he outfoxed the U.S. military and its proxy Afghan forces at the battle of Tora Bora and slipped away, presumably over the border into Pakistan.

Bin Laden’s voice was ostensibly last heard in public in January, when al-Qaida’s propaganda arm released an audio statement from him warning France to withdraw its troops from Afghanistan.

Over the past decade, he regularly mocked the inability of the United States and its allies to find him, issuing dozens of audio and video tapes broadcast on the Internet and on television networks such as al-Jazeera. Despite the frequency of his statements, U.S. intelligence officials were unable to follow the trail back to the al-Qaida leader.

Senior U.S. intelligence officials said bin Laden had remained in control of al-Qaida’s central command and that its leadership council still reported to him, even as his whereabouts were carefully concealed. But they said bin Laden weighed in on major management decisions less frequently than he did prior to 2001 due to security precautions that left him inaccessible for long periods of time.

Bin Laden’s death marks the culmination of a decade-long CIA effort that officials said began to build momentum against al Qaida because of two key factors: a major escalation in the campaign of armed Predator and Reaper drones, and an expanding network of informants that the CIA has assembled from stations inside Afghanistan along the Pakistan border.

Indeed, officials said the two components became mutually reinforcing. Drone strikes not only killed militants associated with al Qaida but sent ripples of anxiety through the network and forced operatives to take substantial risks as they searched for cover.

At the same time, the toll of the drone strikes eroded morale among militant networks, contributing to the agency’s effort to assemble a network of informants independent of of Pakistan’s intelligence services. As the network grew, it fed new intelligence into an elaborate operation used to identify new targets for the drones.

Bin Laden’s death means that he will likely be replaced at the helm of al-Qaida by Ayman al-Zawahiri, the Egyptian surgeon who has long served as his chief deputy. Although he has also been in hiding over the last decade, Zawahiri has been the most visible face and voice of al-Qaida, issuing even more audio and video propaganda statements than bin Laden.

Zawahiri, however, is considered a polarizing figure within the top circles of al-Qaida and has long antagonized Islamic radicals from other factions. U.S. counterterrorism officials predicted he would have a much tougher time preserving unity within al-Qaida and attracting fresh followers.

Bruce Hoffman, a terrorism expert and professor of security studies at Georgetown University, said bin Laden had made preparations for his death ever since 1998 and that al-Qaida almost certainly had a succession plan in place.

 In speeches and statements over the past decade, bin Laden has repeatedly said he looked forward to becoming a martyr for al-Qaida’s cause; some analysts said he probably did not expect to live as long as he did.

“His intention was that with his death, his message would carry greater resonance than in the last years of his life,” Hoffman said.