WASHINGTON — President Barack Obama said Thursday that the United States has reached a “crossroads” in the fight against terrorism and that it is time to redefine and recalibrate a war that will eventually end.

Far from repudiating the controversial use of drones against terrorist targets, Obama defended the tactic as effective, legal and life-saving. But he acknowledged that threat levels have fallen to levels not seen since before the Sept. 11, 2001, attacks, requiring new criteria for the use of lethal force.

Obama used the first major counterterrorism address of his second term to outline newly narrowed guidelines to deploy drones only against targets that pose a “continuing, imminent threat” to the United States and only in cases where avoiding civilian casualties is a “near certainty.”

“As our fight enters a new phase, America’s legitimate claim of self-defense cannot be the end of the discussion,” Obama said. “To say a military tactic is legal, or even effective, is not to say it is wise or moral in every instance.”

In a lengthy, wide-ranging speech at the National Defense University at Fort McNair, Obama used the depiction of a diminished threat environement to make the case for sweeping changes to the counterterrorism landscale, including closing the prison at Guantanamo Bay, Cuba, and finding a U.S. site where military commission trials can be held for eligible detainees.

Among steps to help thin the detention center’s population of 166 people, many of them now on a hunger strike, he called for an end to congressional restrictions on transfers for those cleared to leave, and said he was lifting his own moratorium on the repatriation of several dozen Yemeni prisoners.

But even while declaring that “this war, like all wars, must end,” Obama made clear that other pieces of the nation’s counterterrorism aparatus will remain in place, including targeted killings with drones. He made no mention of ending the CIA’s involvement in the drone campaign.

Obama’s remarks followed a pledge in his State of the Union speech in January to make his counterterrorism policies — particularly about drones — more transparent and accountable to Congress and the American public.

Congressional responses ran the gamut. “The president’s speech today will be viewed by terrorists as a victory,” said Sen. Saxby Chambliss of Georgia, the ranking Republican on the Senate Intelligence Committee. “Rather than continuing successful counterterrorism activities, we are changing course with no clear operational benefit.”

Rep. Adam Schiff, D-Ca., a member of the House Intelligence Committee, said the speech went further in describing the president’s vision of countering a diminished terrorist threat than it did in delineating how it will go about doing so.

“It seemed like the administration is using a two-tiered approach,” Schiff said. “A public speech to set up a broad idea that we’re at a crossroads. And at the same time a more private track which changes the criteria and adds restrictions to the drone program.”

As a result, Schiff said, Obama “raised a number of questions as well as answering some.”

Obama sought to both describe a reduced threat level and avoid dismissing the risk. “Now make no mistake,” he said, “our nation is still threatened by terrorists. From Benghazi to Boston, we have been tragically reminded of that truth.” But rather than “the threat that came to our shores on 9/11,” he said al-Qaida was now “on a path to defeat.”

He outlined a three-fold danger from weakened al-Qaida affiliates, threats to diplomatic facilities and businesses abroad and homegrown extremists.

“This is the future of terrorism,” Obama said. “As we shape our response, we have to recognize that the scale of this threat closely resembles the types of attacks we faced before 9/11.”

Obama said he would not sign any proposed expansion of the 2001 Authorization for the Use of Military Force, which gives the president the power to use military force against al-Qaida. Some lawmakers have argued that the authorization should be revised because it is now used to justify targeted killings against al-Qaeia “associates” in Yemen and Somalia, far removed from the Sept. 11 attacks. Obama said he would work to refine and ulitmately repeal the mandate.

“America is at a crossroads,” he said. “We must define the nature and scope of this struggle, or else it will define us.”

Senior administration officials said some of the specifics of the changes Obama outlined are contained in a classified Presidential Policy Guidance directive on counterterrorism operations that he signed this week.

The officials, who spoke on condition of anonymity to brief reporters before the speech, said that requiring evidence of “a continuing and imminent threat to the United States” to justify a drone strike was more restrictive language than in the past.

The officials said the “near certainty” that civilians would not be hit tightens a previous standard in which civilians have been killed unintentionally. Obama said there was “wide gap” between the estimated thousands of civilian casualties cited by non-governmental organizations and U.S. tallies, but he gave no numbers.

The officials said the classified directive also establishes a “preference” for the military to take the lead in drone operations, which are now conducted by both the armed forces and the CIA. That language suggests a change in course for the CIA, which has carried out hundreds of drone strikes in Pakistan and Yemen, killing an estimated 3,000 militants and civilians.

But the guidance appeared to stop well short of ending the CIA’s drone program, leaving enough room for the agency to continue using a controversial targeting practice known as “signature strikes” in Pakistan.

In his speech, Obama put the conflict in the “Afghan war theater” in a separate category from the fight against al-Qaida “associates” elsewhere. He said the United States would “continue to take strikes against high value al-Qaida targets, but also against forces that are massing to support attacks on coalition forces” until they are withdrawn from Afghanistan at the end of 2014.

The Obama administration, like that of President George W. Bush, has long considered Pakistan’s tribal areas part of the war theater. The carve-out presumably enables the CIA to continue hitting groups in Pakistan that are not part of al-Qaida but regularly attack U.S. forces across the Afghanitan border.

The most immediate impact of the revised drone policy might be in Yemen, a country outside the designated war zone, where the CIA and U.S. military simultaneously have operated armed drones and carried out dozens of strikes over the past two years.

If Obama’s “preference” for the Pentagon to take the lead in such operations is enforced, the CIA could be required to shutter a drone base it built in the desert of southern Saudi Arabia in 2011.

The revised drone policy comes amid a plunge in the overall number of strikes overseas. There were more than 120 strikes in Pakistan and Yemen combined in 2010, but there have been just 23 so far this year.The pace has been slowed by a range of factors, including the diminished number of senior operatives among al-Qaida’s thinned ranks, as well as pauses triggered by diplomatic ruptures in Pakistan and political turmoil in Yemen.

Obama signaled he is open to more fundamental changes, including the creation of a “drone court” that could evaluate targeting criteria and decisions. But he noted the drawbacks of such proposals, and made no commitment of support.

He contrasted the relative precision of drones to the risks of inserting commando teams or relying on the larger and more lethal munitions of conventional airpower.

Even so, his assertions about the drone program’s accuracy may have been undercut by the administration’s disclosure on Wednesday that four U.S. citizens had been killed in strikes over the past four years — and that only one of those, the U.S.-born cleric Anwar al-Awlaki, had been specifically targeted.

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