WASHINGTON » President Barack Obama sought to advance the U.S. beyond the unrelenting war effort of the past dozen years, defining a narrowing terror threat that still imperils the nation but now is defined by smaller networks and homegrown extremists rather than the grandiose plots of Osama bin Laden’s al-Qaida. He defended his controversial drone-strikes program as a linchpin of the U.S. response to the evolving dangers.

In a lengthy address at the National Defense University today, Obama argued that changing threats require changes to the nation’s counterterrorism policies. He implored Congress to close the much-maligned Guantanamo Bay detention center in Cuba and pledged to allow greater oversight of the drone program. But he plans to keep the most lethal efforts with the unmanned aircraft under the control of the CIA.

He offered his most vigorous public defense yet of drone strikes as legal, effective and necessary as terror threats progress.

“Neither I, nor any president, can promise the total defeat of terror,” Obama told his audience of students, national security and human rights experts and counterterror officials. “What we can do — what we must do — is dismantle networks that pose a direct danger, and make it less likely for new groups to gain a foothold, all while maintaining the freedoms and ideals that we defend.”

Obama’s address came amid increased pressure from Congress on both the drone program and the status of the Guantanamo prison. A rare coalition of bipartisan lawmakers has pressed for more openness and more oversight of the highly secretive targeted strikes, while liberal lawmakers have pointed to a hunger strike at Guantanamo in pressing Obama to renew his stalled efforts to close the detention center.

The president cast the drone program as crucial in a counterterror effort that will rely less on the widespread deployment of U.S. troops as the war in Afghanistan winds down. But he acknowledged the targeted strikes are no “cure-all” and said he is deeply troubled by the civilians unintentionally killed.

“For me, and those in my chain of command, these deaths will haunt us as long as we live,” he said. Before any strike, he said, “there must be near-certainty that no civilians will be killed or injured — the highest standard we can set.”

In Pakistan alone, up to 3,336 people have been killed by the unmanned aircraft since 2003, according to the New America Foundation which maintains a database of the strikes. However, the secrecy surrounding the drone program makes it impossible for the public to know for sure how many people have been killed in in strikes, and of those, how many were intended targets.

In an attempt to lift the veil somewhat, the Justice Department revealed for the first time Wednesday that four Americans had been killed in U.S. drone strikes abroad. Just one was an intended target — Anwar al-Awlaki, who officials say had ties to at least three attacks planned or carried out on U.S. soil. The other three Americans, including al-Awlaki’s 16-year-old son, were unintended victims.

Drones aside, some Republicans criticized Obama as underestimating the strength of al-Qaida in his speech and for proposing to repeal the president’s broad authorization to use military force against the nation’s enemies — powers granted to George W. Bush after the Sept. 11, 2001, attacks.

“I believe we are still in a long, drawn-out conflict with al-Qaida,” Sen. John McCain, R-Ariz., told reporters after the speech. “To somehow argue that al-Qaida is on the run comes from a degree of unreality that to me is really incredible. Al-Qaida is expanding all over the Middle East, from Mali to Yemen and all places in between.”

Obama announced new “presidential policy guidelines” on the standards his administration uses when deciding to launch drone strikes. According to an unclassified summary of the guidelines, the U.S. will not strike if a target can be captured, either by the U.S. or a foreign government; a strike can be launched only against a target posing an “imminent” threat, and the U.S. has a preference for military control of the drone program.

However, the CIA will continue to work with the military on the program in Yemen, and control it in Pakistan, given the concern that al-Qaida may return in greater numbers as U.S. troops leave Afghanistan. The military and the CIA currently work side by side in Yemen, with the CIA flying its drones over the northern region out of a covert base in Saudi Arabia and the military flying its unmanned aerial vehicles from Djibouti.

Obama’s advisers said the new guidelines would effectively limit the number of drone strikes in terror zones and pointed to a future decline of attacks against extremists in Afghanistan as the war ebbs. But strikes elsewhere will continue. The guidelines will apply to strikes against both foreigners and U.S. citizens abroad.

Though Obama sought to give more transparency to the drone program, the strikes will largely remain highly secret for the public. Congress has been briefed on every strike that U.S. drones have made outside Afghanistan and Iraq, Obama said, but those briefings are largely classified and held privately.

The president said he was open to additional measures to further regulate the drone program, including creating a special court system to regulate strikes, similar to one that signs off on government surveillance in espionage and terror cases. Congress is already considering whether to set up a court to decide when drones overseas can target U.S. citizens linked to al-Qaida.

While civil rights groups welcomed some of Obama’s steps, they appeared largely unappeased.

“The president still claims broad authority to carry out target killings far from any battlefield, and there is still insufficient transparency,” said Anthony D. Romero, executive director of the American Civil Liberties Union. “We continue to disagree fundamentally with the idea that due process requirements can be satisfied without any form of judicial oversight by regular federal courts.”

Obama was interrupted three times by a woman from the anti-war group Code Pink, who appeared to be protesting both the drone program and the Guantanamo prison. The president said at one point that he was willing to “cut the young lady some slack” because the issues he was addressing were worth being passionate about.

In seeking to close Guantanamo, Obama faces many of the same roadblocks that stymied his efforts to shut the prison when he first took office. Many Republican lawmakers oppose Obama’s efforts to bring some of the detainees to the U.S. to face trial and be held in maximum security American prisons.

But a new hunger strike by prisoners protesting their conditions and indefinite confinement has refocused Obama on efforts to close the detention center. He tried to jumpstart that process today by announcing a fresh push to transfer approved detainees to their home countries and lift a ban on transfers to Yemen.

The end of the Yemen restrictions is key, given that 30 of the 56 prisoners eligible for transfer are Yemeni. Obama halted all transfers to the poor Middle Eastern nation in 2010, after a man trained in Yemen was convicted in a failed bombing attempt of an airliner over Detroit.

In a statement from its embassy in Washington, the Yemeni government said it welcomed the administration’s decision and pledged to “work with the United States to take all necessary steps to ensure the safe return of its detainees.”

Obama acknowledged that the politics of closing Guantanamo are difficult, but he said, “History will cast a harsh judgment on this aspect of our fight against terrorism and those who fail to end it.”

The president said he planned to appoint a special envoy to oversee the prisoner transfers and other efforts to close the prison.

Sen. John McCain, R-Ariz., a leading voice among Republicans, has long advocated closing Guantanamo and pledged Thursday to urge his colleagues to work with Obama to shut the facility. But Rep. Howard P. “Buck” McKeon, the Republican chairman of the House Armed Services Committee, said he was open to a proposal from Obama on the future of Guantanamo Bay. But that plan has to consist of more than political talking points, he said.

“This speech was only necessary due to a deeply inconsistent counterterrorism policy, one that maintains it is more humane to kill a terrorist with a drone than detain and interrogate him at Guantanamo Bay,” McKeon said

This week, the Pentagon asked Congress for more than $450 million for maintaining and upgrading the Guantanamo prison. More than 100 of the prisoners are involved in the hunger strike, and the military earlier this month was force-feeding 32 of them to keep them from starving.

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Associated Press writers Lolita C. Baldor, Kimberly Dozier and Richard Lardner contributed to this report.