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 in Pakistan’s Abbottabad Valley, where bin Laden had been hiding since late last summer. The U.S. team killed the 54-year-old al-Qaida leader after a firefight and “took custody of his body,” Obama said.

“We will be relentless in defense of our citizens and our friends and allies,” a somber Obama said in a 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. It comes at a time of deep political turmoil overseas that is upending long-standing U.S. policy in much of the Muslim world, particularly the Arab Middle East.

The news also drew praise across the usually tall partisan divide.

House Republican leader Eric Cantor, R-Va., said in a statement: “The men and women of our armed forces and intelligence community have fought valiantly for the last decade, and this is a major victory and testament to their dedication.”

Bin Laden was holed up in a two-story mansion in the Bilal area of Abbottabad, about 60 miles north of Islamabad, when four helicopters carrying U.S. anti-terror forces swooped in the early-morning hours of Monday and killed him.

Flames rose from the building that was the apparent target of the raid as it was confirmed that the world’s most wanted fugitive died not in a cave, but in a town best known as a garrison for the Pakistani military. A U.S. official said one of bin Laden’s sons and three others also were killed in the raid, but the official did not name them.

Pakistani officials and a witness said bin Laden’s guards opened fire from the roof of the building, and one of the choppers crashed. The sound of at least two explosions rocked the town. U.S. officials said no Americans were harmed in the raid.

Women and children were taken into custody during the raid, said an official who spoke on condition of anonymity because of the sensitivity of the information.

Abbotabad resident Mohammad Haroon Rasheed said the raid happened about 1:15 a.m. local time.

“I heard a thundering sound, followed by heavy firing. Then firing suddenly stopped. Then more thundering, then a big blast,” he said. “In the morning when we went out to see what happened, some helicopter wreckage was lying in an open field.”

The development comes 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.

Public support for the war and Obama’s management of it has deteriorated steadily since then, in part because many Americans are uncertain of the long-term U.S. goal in the nearly decade-old conflict and how much progress international forces have made there over that time. More Americans disapprove of Obama’s management of the war than approve of it, according to a recent Washington Post-ABC News poll.

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 almost certainly will 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 first received intelligence of bin Laden’s possible whereabouts in August, and gave the order Sunday for the operation that ended in his death.

“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.”

With the 10th anniversary of the Sept. 11 attacks approaching this year, bin Laden’s assassination could benefit Obama domestically even more than the capture of former Iraqi dictator Saddam Hussein helped propel former President George W. Bush to re-election in 2004.

Although former Bush officials were quick to declare bin Laden’s death a military victory that transcended party lines, it represented the culmination of Bush’s promise, never fulfilled during his time in office, to capture the al-Qaida leader “dead or alive.”

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, complicated legal battle if the al-Qaida leader was taken into U.S. custody alive.

With the military brig at Guantánamo 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 the 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 10 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 attacks 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.

His 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.

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 before 2001 due to security precautions that left him inaccessible for long periods of time.

His death marks the culmination of a decadelong 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 Pakistan’s intelligence services. As the network grew, it fed new intelligence into an elaborate operation used to identify new targets for the drones.

Yet, counterterrorism cooperation between the United States and Pakistan had been at one of its lowest ebbs in the decade since the Sept. 11 attacks.

Pakistan has threatened to expel CIA operatives from the country and to force the United States to suspend drone strikes against insurgents and al-Qaida targets — threats that the CIA has brushed aside in increasingly defiant fashion. But Obama said Sunday night that he called Pakistan’s President Asif Ali Zardari to inform him of the operation.

The CIA has carried out 23 drone strikes this year, including one the day after Pakistan’s intelligence chief had visited CIA headquarters to demand changes in the relationship. Another strike came one day after Pakistan released a CIA security operative who had been jailed after fatally shooting two Pakistani men in Lahore.

What effect bin Laden’s death will have on al-Qaida’s operations is uncertain.

Bin Laden’s death means he likely will be replaced at the helm by Ayman al-Zawahiri, an Egyptian surgeon who has long served as his chief deputy. Although he also has been in hiding over the past 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 long has 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.

Bin Laden repeatedly has 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.

“Zawahiri becomes the obvious heir apparent, and I think he’s been running the organization in any event,” Hoffman said. “The question is, how effective will Zawahiri be in filling bin Laden’s shoes?”