Mehsud was Pakistan's public enemy No. 1 and its most feared militant, responsible for a string of bombings and assassination attempts. But while Mehsud carried out strikes against U.S. forces overseas and had a $5 million bounty on his head, he had never been the top priority for U.S. airstrikes, something that at times rankled Pakistan.
"The decision was made to find him, to get him and to kill him," a senior U.S. intelligence official said, recalling weeks and months of "very tedious, painstaking focus" before an unmanned CIA aircraft at his father-in-law's house near Pakistan's border with Afghanistan.
It was not the first airstrike on Obama's watch, but it marked the first major victory in his war on terrorism, a campaign the administration believes can be waged even more aggressively than its predecessor's. Long before he went on the defensive in Washington for his handling of the failed Christmas Day airline bombing, Obama had widened the list of U.S. targets abroad and stepped up the pace of airstrikes.
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Advances in spy plane technology have made that easier, as has an ever-improving spy network that helped locate Mehsud and other terrorists. These would have been available to any new president. But Obama's counterterrorism campaign also relies on two sharp reversals from his predecessor, both of which were political gambles at home.
Obama's national security team believed that the president's campaign promise to pull U.S. troops out of Iraq would have a side benefit: freeing up manpower and resources to hunt terrorists in Pakistan and Afghanistan. Intelligence officials, lawmakers and analysts say that approach is showing signs of success.
Obama also has sought to reach out to Islamic allies and tone down U.S. rhetoric, a language shift that critics have argued revealed a weakness, in an effort to win more cooperation from countries like Yemen and Pakistan.
For example, though Pakistan officially objects to U.S. airstrikes within its border, following the Mehsud strike, the U.S. has seen an increase in information sharing from Pakistani officials, which has helped lead to other strikes, according to the senior law enforcement official. He and other current and former officials spoke on condition of anonymity to discuss sensitive security matters.
Pakistan's cooperation is key to U.S. counterterrorism efforts because much of the best intelligence still comes from Pakistan's intelligence agency. Ensuring that cooperation has been a struggle for years, in part because Pakistan wants greater control over the drone strikes and its own fleet of aircraft, two things the U.S. has not allowed.
"The efforts overseas are bearing fruit," said Sen. Lindsey Graham, R-S.C., a strident critic of Obama's domestic counterterrorism policies who said Obama has at times shown himself even more aggressive than Bush in his use of force overseas. "I give them generally high marks for their efforts to capture and kill terrorists in Pakistan, and they're pushing the envelope in Yemen."
CIA drones, the remote-controlled spy planes that can hunt terrorists from miles overhead, are responsible for many of the deaths. Drone strikes began increasing in the final months of the Bush administration, thanks in part to expanded use of the Reaper, a newer generation aircraft with better targeting systems and greater, more accurate firepower.
Obama has increased their use even further. A month after Mehsud's death, drone strikes in Pakistan killed Najmiddin Jalolov, whose Islamic Jihad Union claimed responsibility for bombings in 2004 at U.S. and Israeli embassies in Uzbekistan. Senior al Qaeda operatives Saleh al-Somali and Abdallah Sa'id were killed in airstrikes in December. And Mehsud's successor at the Pakistani Taliban, Hakimullah Mehsud, died following an attack last month.
Intelligence officials and analysts say the drawdown of troops in an increasingly stable Iraq is part of the reason for the increase in drone strikes. The military once relied on drones for around-the-clock surveillance to flush out insurgents, support troops in battle and help avoid roadside bombs.
With fewer of those missions required, the U.S. has moved many of those planes to Afghanistan, roughly doubling the size of the military and CIA fleet that can patrol the lawless border with Pakistan, officials said.
"These tools were not Obama creations, but he's increased their use and he has shifted the U.S. attention full front to Afghanistan," said Thomas Sanderson, a defense analyst and national security fellow at the Center for Strategic and International Studies.
The Obama administration has also benefited from stepped-up cooperation with officials in Osama bin Laden's ancestral homeland of Yemen. Authorities there killed 30 suspected militants in airstrikes in December closely coordinated with U.S. intelligence agencies.
Yemen has had a sometimes rocky relationship with the U.S. and was perceived to have an on-again-off-again approach to fighting terrorism, but officials in Washington are cautiously optimistic about a newly strengthened relationship.
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Abdullah al-Saidi, Yemen's ambassador to the United Nations, said his country has always been committed to fighting terrorism. But in a fragmented country beset by a growing al Qaeda presence, a rebellion in the north and a secessionist movement in the south, it wasn't always easy for the government to openly align with the United States.
Washington is trying to make it easier with the promise of more money. But perhaps more important, al-Saidi said, were overtures such as Obama's June 2009 speech in Cairo, where he sought a "new beginning" with the Muslim world.
Obama has also abandoned terms like "radical Islam" and "Islamo-fascism," rhetoric that was seen as anti-Muslim by many in the Arab world and which al-Saidi said made it harder for governments to openly cooperate with Washington.
"Just the notion of not equating Islam with terrorism, there is a lot of good will toward him," al-Saidi said. "For the public, it's easier to say, 'Well, it's no longer a hostile power as it used to be."'
Such international successes have largely been drowned out by the controversy that followed the failed bombing of a U.S.-bound airliner on Dec.25. When the FBI read suspected Nigerian bomber Umar Farouk Abdulmutallab his rights and charged him in federal court, Republicans accused Obama of not understanding the country is at war.
"They're trying to be tougher than Bush overseas but different from Bush at home," Graham said. "It doesn't make a lot of sense. They really got the right model for Pakistan and Yemen, but they're really tone deaf at home."
After Obama missed his own , Cuba, and backtracked on a plan to prosecute Sept. mastermind Khalid Sheikh Mohammed in a New York courthouse, Republicans saw the airliner case as an opportunity to renew questions about Obama's national security credentials, Republican strategist Kevin Madden said.
Madden said that Obama's stepped-up strategy overseas does not resonate with voters, and Republicans gain little in an election year by acknowledging where they agree with the White House strategy.
"National security politics is driven by events more than it's driven by long-term trends," he said.
Or, as Graham put it: "What resonates with people is what happens in Detroit, more than what happens on the Afghanistan-Pakistan border."
The White House says it sees no conflict between broadening the attacks overseas and sticking with the U.S. judicial system at home, where hundreds of people have been convicted on terrorism charges since the attacks of Sept. 11, 2001.
"The president believes that we need to use all elements of American power to defeat al Qaeda, including the strength of our military, intelligence, diplomacy and American justice," said Ben Rhodes, White House deputy national security adviser. "We only weaken ourselves when we fail to use our full arsenal."