(Reuters) - A pivotal moment in the long, tortuous quest to find Osama bin Laden came years before U.S. spy agencies discovered his hermetic compound in Abbottabad, Pakistan.

In July 2007, then Senator Barack Obama's top foreign policy advisers met in the modest two-room Massachusetts Avenue offices that served as his campaign's Washington headquarters. There, they debated the incendiary language Obama would use in an upcoming speech on national security, according to a senior White House official.

Pakistan was a growing worry. A new, highly classified intelligence analysis, called a National Intelligence Estimate, had just identified militant safe havens in Pakistan's border areas as a major threat to U.S. security. The country's military leader, Pervez Musharraf, had recently cut a deal with local tribes that effectively eased pressure on al Qaeda and related groups.

Days after the Washington meeting, candidate Obama told an audience at the Woodrow Wilson International Center for Scholars: "If we have actionable intelligence about high-value terrorist targets and President Musharraf won't act, we will."

It was the most carefully crafted sentence in the speech, a statement no U.S. leader had ever made. (Text of Obama's speech: link.reuters.com/weg59r)

In the two weeks since President Obama made good on that threat -- in fact, bested it by declining to give Pakistan a chance to act first -- reams have been written about the painstaking detective hunt that led to bin Laden.

But Reuters interviews with two dozen current and former senior intelligence, White House and State Department officials reveal another side of the story.

The 13-year quest to find and eliminate bin Laden, from the November 1998 day he was indicted by a federal grand jury for his role in the East Africa embassy bombings, was filled with missteps, course adjustments and radical new departures for U.S. security policy. It ultimately led to a fortified compound in a little known Pakistani city named after a long-dead British major.

Even with bin Laden buried at sea, the changes to U.S. security policy could linger for years, or decades.

The mission to destroy bin Laden, and his network, sparked the creation of a chillingly bureaucratic process for deciding who would be on "kill lists," authorized for death at the hands of the CIA. It revolutionized the use of pilotless drones to find and attack militants; drove the controversially brutal treatment of detainees in U.S. custody; and brought the United States and Pakistan closer together, then wrenched them apart.

(Even in ordering the risky Navy SEAL raid on May 1, Obama made allowances for Pakistan's sensitivities. The raid was carried out by the U.S. military but under CIA legal authorities and command, partly for deniability if something went wrong and partly because the United States is not at war with Pakistan, a U.S. official said.)

But there was one constant in the search for bin Laden. On September 17, 2001, six days after the 9/11 attacks, President George W. Bush issued a still-classified "finding" that gave the CIA "lethal authorities" to deal with the al Qaeda leader and his top lieutenants. Ever since, there was an expectation -- even a preference -- that bin Laden would be killed, not captured, Bush and Obama administration officials said.

The same day that Bush signed the directive, he publicly declared bin Laden was wanted "dead or alive."

Numerous officials said they knew of no explicit command that bin Laden was not to be taken alive. When he ordered the SEAL raid, Obama had on his desk a written protocol for what would happen if the al Qaeda chief were captured and removed from Pakistan to an unnamed U.S. military installation, the senior White House official said.

But it was vaguer than the rest of the operational plan, and the expectation among most of the people who planned and executed the mission was that bin Laden would be killed. If bin Laden had surrendered, Obama's senior advisers "would have to reconvene and make a decision about what to do with him," said one official, who like many requested anonymity to discuss sensitive national security matters. "It was intentionally left to be decided after the fact."

Richard Armitage, who was deputy secretary of state in Bush's first term, voiced the view that prevailed through two presidencies. "I think we took Osama bin Laden at his word, that he wanted to be a martyr," Armitage told Reuters.

The U.S. government, he said, would do all it could to help bin Laden realize that goal.

RABBIT HOLES AND WRONG TURNS

The hunt for bin Laden turned out to be riddled with dead ends, wrong turns and long, desolate periods of frustration.

The 9/11 attacks would push the Bush administration into a war in Iraq that critics -- including candidate Obama -- denounced as a dangerous diversion from al Qaeda and its Afghanistan/Pakistan nexus. Interrogation techniques such as "waterboarding," a form of simulated drowning, were used on a handful of suspects deemed most dangerous, sparking a debate -- it erupted again on May 2 -- over the best way to fight terrorism.

In Afghanistan's Tora Bora mountains in December 2001, U.S. special forces came close to bin Laden -- perhaps within 2,000 meters, according to the published recollections of a former U.S. Army special forces commander who uses the pseudonym "Dalton Fury."

Opting to rely on local Afghan allies, the United States declined to send in the 1,500 U.S. Army Rangers needed to block bin Laden's escape route.

It would be more than nine years before U.S. special forces would get that close again.

In the intervening years, "there were a lot of empty rabbit holes down which we pursued and ultimately didn't find any results. It was very frustrating," said Juan Zarate, a top White House counter-terrorism aide from 2005-2009. "I always had a mantra that I used for myself, both not to get too discouraged and also with the counter-terrorism community, which is: these guys are not ghosts. They are flesh and blood and can be found and we'll find them."

With virtually no hard knowledge, U.S. counter-terrorism officials said they assumed bin Laden was hiding in the mountainous, lawless Afghan-Pakistan border region. But it's now believed that after Tora Bora, he spent some time in Afghanistan's eastern Kunar province, crossed the border into Pakistan in late summer or fall 2002, moved to a Pakistani village in 2003 for a couple of years, and hid in plain sight in Abbottabad beginning in 2005 or 2006.

Yet even in deadly U.S. failures, there were small breakthroughs.

On February 4, 2002, a Predator drone struck a group of men in Arab dress in the Zawar Kili area of eastern Afghanistan. Among them was a tall man to whom others were acting deferentially, U.S. officials said at the time.

It turned out not to be bin Laden. Reports quoted local residents saying it was a group of villagers collecting scrap metal. But before the episode was over, U.S. intelligence agencies had received, with help from the Saudi government, a DNA sample from bin Laden's extended family that would clinch identification if he were ever found.

FROM CAPTURE TO KILL

It was President Bill Clinton who launched the hunt for bin Laden. After the 1998 bombings of U.S. embassies in Kenya and Tanzania, Clinton signed what some former U.S. officials called a "covert action finding" authorizing CIA operations against al Qaeda, then regarded as a marginal Islamic militant faction with an eccentric, Saudi-born leader.

But some Clinton aides, led by attorney general Janet Reno, were concerned about the legality of killing bin Laden, former top intelligence and counter-terrorism officials said. Clinton's orders permitted U.S. forces to kill bin Laden in self-defense, but the prime directive was to capture him and bring him to justice in the United States.