In authorizing the killing of an American citizen affiliated with al-Qaeda, President Obama has starkly affirmed his willingness to use high-risk, violent tactics against declared enemies.

The drone strike against Anwar al-Aulaqi, which the White House on Friday refused to acknowledge publicly, burnishes Obama’s national security credentials as he heads into an election year when the strength of his leadership will undoubtedly be questioned.

Carried out in northern Yemen, the U.S. attack also underscores Obama’s willingness to operate outside the defined U.S. combat zones in Iraq and Afghanistan in waging what was once known as the global war on terror. Obama stopped using that term when he took office, arguing that it overstated the strength and scope of the U.S. enemy.

“This is further proof that al-Qaeda and its affiliates will find no safe haven anywhere in the world,” Obama told a mostly military audience Friday at a farewell ceremony for Adm. Mike Mullen, the outgoing chairman of the joint chiefs of staff. “We will be determined, we will be deliberate, we will be relentless, we will be resolute in our commitment to destroy terrorist networks that aim to kill Americans.”

But in relying on drones far more than his predecessor did in hunting down alleged terrorists, Obama is intensifying the war with al-Qaeda across a broader geographic area — and stretching the boundaries of its legal rationale.

Although citizenship is not a factor in determining whether a person can be lawfully killed under the laws of war, Obama has taken a clear step beyond the Bush war on terror in killing Aulaqi, who was never indicted for his alleged acts. A second U.S. citizen, Samir Khan, also died in the attack.

The Justice Department had written a secret memo authorizing the targeting of Aulaqi, following a broad review of the legal issues raised by a strike on a U.S. citizen, according to administration officials who spoke on condition of anonymity. They said none of the lawyers involved in the process, from across the government, dissented.

A former constitutional law lecturer, Obama declined Friday to explain the legal justification for the strike. For a president who promised a more rigorous adherence to the law in his national security policy — as well as a more transparent administration — his silence drew concern and charges of hypocrisy.

“The president clearly has the background and intelligence to explain the legal theory justifying the attacks,” said Kenneth Roth, executive director of Human Rights Watch. “His refusal to do so is a deliberate decision, and it is unwise because it undermines the justification for the attack and makes it open to misinterpretation by other governments seeking to expand their own operations against perceived terrorists.”

Obama emerged from a crowded Democratic field in 2008 on the basis, in part, of his opposition to the Iraq war.

Although he pledged to escalate operations against al-Qaeda if elected, Obama was perceived by many voters as the anti-war candidate, an image he buttressed with a sharp critique of President George W. Bush’s national security regimen.

Obama called Bush’s detention and interrogation policies inconsistent with U.S. law and values, at some political risk given his foreign policy inexperience. A Washington Post-ABC News poll taken six months after his inauguration found that 57 percent of the public approved of his handling of the issue.

Since then, Obama has more than doubled the number of U.S. troops in Afghanistan, joined other nations in a military operation to protect civilians in Libya and ultimately oust longtime leader Moammar Gaddafi, and authorized a commando raid deep inside Pakistan that killed Osama bin Laden.

He has also significantly expanded the use of drones against al-Qaeda leaders and foot soldiers. In Pakistan alone, the United States has carried out 227 drone strikes since Obama took office, nearly five times more than Bush conducted during his eight-year tenure.

According to the New America Foundation, the Obama administration has killed at least 1,100 combatants in those strikes, also a nearly fivefold increase from the Bush years. Those figures do not include civilian deaths that resulted from the remotely controlled attacks.

As his overall approval rating has sunk, Obama has seen a rise in support for his handling of national security. A Post-ABC News poll published last month showed that 62 percent approve of how he has managed the terrorist threat.

“In international affairs, he will take risks,” said John Podesta, president of the Center for American Progress, a think tank close to the administration. “The risks Bush took were bad bets because they were so influenced by ideology. Obama takes risks based on analysis and calculations.”

At a time of deep economic uncertainty, public support for Obama’s national security policies, including a steady troop drawdown from Iraq, has not improved his overall political standing on the eve of a difficult election year.

Although his approval rating jumped 9 percentage points immediately after bin Laden’s killing in May, the boost evaporated within weeks. In a New York Times-CBS News poll published last month, only 2 percent of respondents listed terrorism and national security as “the most important problem facing the country.”

But Michael E. O’Hanlon, a senior foreign policy fellow at the Brookings Institution, said Obama has likely benefited from the intangible effect that diplomatic and military success can bring to a presidency, especially a Democratic one.

“Polls often miss the fact that if you seem strong on national security, it makes you look presidential more broadly, so it helps your image as a leader,” O’Hanlon said. “Unlike his predecessors in the Democratic Party, Obama’s not really accused of being weak and I think that’s a huge advantage for him heading into the campaign.”

In his remarks Friday, Obama called Aulaqi’s killing “another significant milestone in the broader effort to defeat al Qaeda and its affiliates.” He did not mention Aulaqi’s U.S. citizenship; administration officials stressed throughout the day that he is also a citizen of Yemen.

Many on the left expressed concern that Obama has taken the war against al Qaeda into territory where Bush did not venture. Rep. Dennis J. Kucinich (D-Ohio) said in a statement that “the administration has a crossed a dangerous divide and set a dangerous precedent for how the United States handles terrorism cases.”

“Mr. al-Aulaqi’s allegedly violent rejection of America was not acceptable in any way,” Kucinich said. “Neither is it acceptable to trample the Constitution through extrajudicial killings.”

The operation also drew different reactions from Obama’s Republican rivals with Gov. Rick Perry of Texas praising the operation and Rep. Ron Paul, also of Texas, criticizing the president for killing a U.S. citizen without due process.

“Nobody knows if he ever killed anybody,” Paul said, according to a report in the Wall Street Journal. “If the American people accept this blindly and casually . . . I think that’s sad.”

Staff researcher Julie Tate and polling analyst Scott Clement contributed to this report.