WASHINGTON - Attorney General Eric Holder was just months into the job when he announced plans to prosecute the accused mastermind of the Sept. 11 attacks and other alleged co-conspirators in a New York courtroom, rather than through the Guantanamo Bay military commission process.

It was an audacious idea, but immediately beset by political opposition and public safety concerns. The Obama administration's eventual decision to walk away from the proposal was a stinging defeat for Holder - and a vivid reminder of the complexities of the legal fight against terrorism.

Holder, who announced Thursday that he would step down once a successor is confirmed, took office determined to turn the page from Bush administration policies that authorized harsh interrogation techniques against suspected terrorists. But he will leave with a mixed record of national security decisions that have drawn their own scrutiny and disappointed those who felt he didn't go far enough to distance the Justice Department from past practices.

The department points to hundreds of terrorism-fighting successes during Holder's tenure, including prosecutions of plots to explode a bomb in Times Square and on a Detroit-bound airliner.

But also under his watch, the department authorized targeted drone strikes against Americans abroad, subpoenaed journalists' telephone records in leak investigations and defended in court the government's bulk collection of millions of Americans' telephone records.

"It's a complex legacy that I think is somewhat disappointing on national security," said Seton Hall University law professor Jonathan Hafetz, a former senior attorney with the American Civil Liberties Union.

Hafetz said he believed Holder, attorney general since the start of Obama's presidency in 2009, had overall been "too supportive of executive power and insufficiently protective of civil liberties."

Stephen Vladeck, an American University law school professor, credited the attorney general for overseeing a large number of terrorism prosecutions. But in a national security world where much is classified, Vladeck said, "the visible stuff doesn't look as good as the stuff we don't see."

"His legacy is one, that, unfortunately is going to be marked by the more visible moments, which aren't as flattering," Vladeck said.

When Holder took over, he quickly signaled a new direction from the previous Republican administration.

During Senate hearings on his nomination, for instance, Holder defined waterboarding as torture. He later announced an investigation into CIA interrogation methods of terrorist detainees; that three-year inquiry ended without criminal charges.

He similar pushed a shift away from the military commission system the Bush administration enacted to prosecute suspected terrorists at Guantanamo Bay in Cuba. In November 2009, the attorney general announced that the Justice Department would seek to prosecute five detainees, including professed Sept. 11 mastermind Khalid Sheikh Mohammed, in federal court in New York and pursue the death penalty.

Critics, mostly Republicans, immediately argued the federal courts were ill-equipped for such a trial and expressed alarm that terrorists would be brought to American soil.

The White House in 2011 shelved the idea, which was seen as a significant step toward closing the Guantanamo detention facility.

Despite that setback, Holder remained firm in supporting civilian courts for suspected terrorists, and his Justice Department has won many convictions before American juries.

He has said his position was vindicated by the fact that, in the years since the Guantanamo proposal was raised and spiked, Osama bin Laden's son-in-law, Sulaiman Abu Ghaith - the highest-ranking al-Qaida figure to face trial on U.S. soil since the attacks - was captured, convicted in New York and sentenced to life in prison. The military commission case against Mohammed, meanwhile, remains stalled in Guantanamo by pretrial wrangling.

"I have great respect for him, and I know how hard he took not being able to try Khalid Sheikh Mohammed in court," said Laura Murphy, director of the ACLU's Washington legislative office.

Yet civil liberties advocates who appreciated Holder's efforts to afford legal protections for suspected terrorists were nonetheless frustrated when he later defended the administration's legal justification to kill American-born Anwar al-Awlaki in Yemen. They challenged the government's constitutional authority to kill one of its citizens and its refusal, until recently, to release the memo authorizing the strike.

An unmanned U.S. drone killed al-Awlaki in Yemen in 2011. U.S. officials considered him to be an inspirational leader of al-Qaida, and they linked him to the planning and execution of several attacks targeting American and Western interests, including the 2009 attempt on Christmas Day on a Detroit-bound airliner.

The ACLU, in a statement on Holder's departure, cited the case as among those where "we've had profound disagreements" with Holder.

Additional scrutiny came from the administration's crackdown on news media reporting involving national security matters.