Osama bin Laden, chief architect of the Sept. 11, 2001 terrorist attacks, is dead. (AFP photo) (-/AFP)

The phone rang in the early morning hours at Chequers, the country estate of British prime ministers northwest of London. President Obama — before he addressed the American people — was calling to inform Britain, the closest ally of the United States in the war in Afghanistan, that Osama bin Laden was dead.

In a nation that saw dozens killed in an al-Qaeda-inspired attack on the London transit system in 2005, Prime Minister David Cameron responded by publicly thanking Obama and U.S. forces for tracking bin Laden down. He said in a national broadcast that the al-Qaeda leader’s death marked “a massive step forward” in the war against extremists. But, following the U.S. lead, Britain also put its embassies overseas on high alert for possible retribution, and Cameron warned of the need for “vigilance in the weeks ahead.”

Washington’s European allies largely echoed Cameron’s mixture of optimism and caution on Monday, with leaders in Berlin, Paris and other capitals hailing the U.S. operation in Pakistan. In a region that has come under direct threat from al-Qaeda, bin Laden’s death appeared to restore a measure of confidence in U.S. intelligence and military capabilities. But like Obama, European leaders tempered their words with reminders that al-Qaeda remains an active and definitive threat.

“The forces of peace achieved a victory last night,” German Chancellor Angela Merkel said in a statement. “But that doesn’t mean international terrorism has been beaten yet.”

Less than a week after al-Qaeda’s North Africa branch released tapes of four slain French hostages — an attempt to pressure the government in Paris to withdraw troops from Afghanistan — President Nicholas Sarkozy’s office issued a statement saying that for bin Laden’s thousands of victims, “justice has been done.”

Sarkozy hailed what he described as U.S. tenacity in hunting down bin Laden a decade after al-Qaeda’s attacks in New York and Washington. He said the terrorist leader’s slaying marks “a major event in the worldwide struggle against terrorism” but cautioned that France and the world should not let down their guard because the threat remains.

Nevertheless, analysts were divided over how bin Laden’s death would affect allied commitment to the highly unpopular war in Afghanistan.

Particularly at a time when Britain and France are stretching their militaries in Libya, some analysts said there would be a temptation to see Monday’s operation as a good reason to accelerate a withdrawal.

Daniel Korski, senior policy fellow at the European Council on Foreign Relations, said the region’s powers had initially engaged in Afghanistan under the banner of “the hunt for bin Laden.” When that failed, he said, the mission there “was broadened out to a more traditional peacekeeping role. It was about children and schools, about giving girls opportunities they never had before.” But now, he said, bin Laden’s sudden end may give the leaders the cover they need to speed withdrawal and still claim victory.

Other analysts resoundingly disagree. Robin Niblett, director of the London-based think tank Chatham House, said the caution expressed by leaders across the region underscored the sense in Europe of a lingering threat from al-Qaeda. In addition, he noted that Britain — by far the largest European contributor of troops to Afghanistan — has clearly stated that the Taliban, not al-Qaeda, remains the major foe. “I don’t think this changes the strategy or the timetable,” Niblett said. “They want to get out around 2014, and they don’t want to leave behind a failed state. Whether Osama bin Laden is alive or dead is not going to change that.”

Across the region, perhaps nowhere was the reaction to bin Laden’s death more significant than in Britain, which more than any other nation has stood beside U.S. troops in the military actions that followed the Sept. 11, 2001, attacks.

The news spread in the early morning of the May Day bank holiday, dominating national television. British tabloids rushed stories online, such as the Daily Mail’s “The last lair of a mass murderer” and the Sun’s more straightforward “Osama Bin Laden Shot Dead.”

British observers appeared captivated by the scenes of spontaneous celebration in front of the White House and at Ground Zero in New York. Writing for the Telegraph newspaper, Tim Stanley, an American history professor at Royal Holloway College, described the celebrations as having overtones of the “wild West.” He wrote: “All of this has the feeling of a peculiarly American kind of justice, the kind that no other country in the world would probably either execute, or enjoy, quite so well.”

The BBC found the celebrations equally fascinating, inviting Christopher Meyer, the British ambassador to the United States during the 2001 attacks, to explain the U.S. reaction to viewers. The United States, Meyer said, “is a much more overtly patriotic country than any European country I know, so this reaction is to be expected.” The death of bin Laden showed that if “you do something so outrageous and harrowing to [the Americans], they will never give up the hunt.”

Expressing his “heartfelt gratitude” to Obama on Monday, former prime minister Tony Blair reminded the nation that Sept. 11 was also the worst-ever terrorist attack against British civilians — 67 Britons died that day in the United States — with the country suffering more casualties than any other foreign nation.

Few knew that more than Patricia Bingley, whose son, Kevin Dennis, 43, was killed in the New York attack. On bin Laden’s death, she told the BBC, “I really began to believe that it would never happen, not in my lifetime, as I am getting older,” she said. “I just wanted justice for my son. And, thank goodness, I have now got it. And it has brought closure.”

Special correspondent Karla Adam contributed to this report.