President Obama on Thursday said he will impose stricter limitations on the use of drone strikes against suspected terrorists and renewed his pledge to close the U.S. prison at Guantanamo Bay, declaring America is at a "crossroads" in its fight against terrorism.

In the most expansive detailing of his counterterrorism policies, the president outlined the legal rationale for remote-controlled, targeted killings, which increased dramatically under his watch and included the deaths of four U.S. citizens. Under his new rules, drones would be used only when the target presents a "continuing and imminent threat" to Americans and when the United States can determine with "near certainty" that no civilians would be killed.

Obama also wants the Defense Department, rather than the CIA, to play a larger role in operating the drones. And the president alluded to establishing an independent court to review possible drone strikes.

"We must define the nature and scope of this struggle, or else it will define us," Obama said during an hour-long speech at the National Defense University in D.C.

Addressing one of the glaring unfulfilled promises of his presidency, Obama lifted his self-imposed moratorium on transferring Gitmo detainees to Yemen, appointed a new State Department envoy to oversee transfers to third-party countries and vowed to bring terrorists to justice in American courts. Nearly 90 Guantanamo prisoners have been cleared for transfer but are still being held indefinitely in Cuba.

However, Obama still needs Republican support to shutter the prison — a prospect that many conservatives dismissed on Thursday.

The president is framing his new strategy as a break from the post-Sept.11 national security policies of former President George W. Bush, who said the United States was engaged in an ongoing "war on terror."

Republicans immediately pounced on Obama's new blueprint, calling it detrimental to national security.

"The president's speech today will be viewed by terrorists as a victory," said Sen. Saxby Chambliss, R-Ga. "Rather than continuing successful counterterrorism activities, we are changing course with no clear operational benefit."

Added Sen. John McCain, R-Ariz., "To somehow argue that al Qaeda is somehow 'on the run' comes from a degree of unreality that is really quite incredible. Al Qaeda is expanding all over the Middle East, from Mali to Yemen and all places in between."

Though his administration acknowledged Wednesday that drones had killed four U.S. citizens living abroad, Obama said Thursday it would be unconstitutional "for the government to target and kill any U.S. citizen."

Obama's own political supporters have hammered him for continuing national security policies he derided as a candidate — and it was a progressive protester during his speech Thursday heckling him about ongoing hunger strikes at Guantanamo.

"I'm willing to cut the young lady who interrupted me some slack because it's worth being passionate about," Obama said.

With revelations that his Justice Department had secretly monitored reporters in Washington, Obama vowed to do more to ensure journalists could work without fear of prosecution. He instructed Attorney General Eric Holder to report back to him by July 12 on guidelines for investigating reporters.

"I am troubled by the possibility that leak investigations may chill the investigative journalism that holds government accountable," Obama said. "Journalists should not be at legal risk for doing their jobs."