By the time Mahmoud Jibril cleared customs at Le Bourget airport and sped into Paris, the U.S. secretary of state had been waiting for hours. But this was not a meeting Hillary Clinton could cancel. Their encounter could decide whether America was again going to war.

In the throes of the Arab Spring, Moammar Gadhafi was facing a furious revolt by Libyans determined to end his quixotic 42-year rule. The dictator’s forces were approaching Benghazi, the crucible of the rebellion, and threatening a blood bath. France and Britain were urging the United States to join them in a military campaign to halt Gadhafi’s troops, and now the Arab League, too, was calling for action.

President Barack Obama was deeply wary of another military venture in a Muslim country. Most of his senior advisers were telling him to stay out. Still, he dispatched Clinton to sound out Jibril, a leader of the Libyan opposition. Their late-night meeting on March 14, 2011, would be the first chance for a top U.S. official to get a sense of whom, exactly, the United States was being asked to support.

In her suite at the Westin, Clinton and Jibril, a political scientist with a doctorate from the University of Pittsburgh, spoke at length about the fast-moving military situation in Libya. But Clinton was clearly also thinking about Iraq, and its hard lessons for U.S. intervention.

Did the opposition’s Transitional National Council really represent the whole of a deeply divided country, or just one region? What if Gadhafi quit, fled or was killed — did they have a plan for what came next?

“She was asking every question you could imagine,” Jibril recalled.

Clinton was won over. Opposition leaders “said all the right things about supporting democracy and inclusivity and building Libyan institutions, providing some hope that we might be able to pull this off,” said Philip H. Gordon, one of her assistant secretaries. “They gave us what we wanted to hear. And you do want to believe.”

Her conviction would be critical in persuading Obama to join allies in bombing Gadhafi’s forces. In fact, Obama’s defense secretary, Robert M. Gates, would later say that in a “51-49” decision, it was Clinton’s support that put the ambivalent president over the line.

The consequences would be more far-reaching than anyone imagined, leaving Libya a failed state and a terrorist haven, a place where the direst answers to Clinton’s questions have come to pass.

This is the story of how a woman whose Senate vote for the Iraq war may have doomed her first presidential campaign nonetheless doubled down and pushed for military action in another Middle Eastern country. As she once again seeks the White House, campaigning in part on her experience as the nation’s chief diplomat, an examination of the intervention she championed shows her at what was arguably her moment of greatest influence as secretary of state. It is a working portrait rich with evidence of what kind of president she might be, and especially of her expansive approach to the signal foreign-policy conundrum of today: whether, when and how the United States should wield its military power in Syria and elsewhere in the Middle East.

From the earliest days of the Libya debate, Clinton was a diligent student and unrelenting inquisitor, absorbing fat briefing books, inviting dissenting views from subordinates, studying foreign counterparts to learn how to win them over. She was a pragmatist, willing to improvise — to try the bank-shot solution. But above all, in the view of many who have watched her up close, her record on Libya illustrates how, facing a national-security or foreign-policy quandary, she was inclined to act — in marked contrast to Obama’s more reticent approach.

Anne-Marie Slaughter, her director of policy planning at the State Department, notes that in conversation and in her memoir, Clinton repeatedly speaks of wanting to be “caught trying.” In other words, she would rather be criticized for what she has done than for having done nothing at all.

“She’s very careful and reflective,” Slaughter said. “But when the choice is between action and inaction, and you’ve got risks in either direction, which you often do, she’d rather be caught trying.”

The New York Times’ examination of the intervention offers a detailed accounting of how Clinton’s deep belief in America’s power to do good in the world ran aground in a tribal country with no functioning government, rival factions and a staggering quantity of arms. The Times interviewed more than 50 American, Libyan and European officials, including many of the principal actors. Virtually all agreed to comment on the record. They expressed regret, frustration and in some cases bewilderment about what went wrong and what might have been done differently.

Was the mistake the decision to intervene in the first place, or the mission creep from protecting civilians to ousting a dictator, or the failure to send a peacekeeping force in the aftermath?

Clinton declined to be interviewed. But in public, she has said it is “too soon to tell” how things will turn out in Libya and has called for a more interventionist approach in Syria.

Libya’s descent into chaos began with a rushed decision to go to war, made in what one top official called a “shadow of uncertainty” as to Gadhafi’s intentions. The mission inexorably evolved even as Clinton foresaw some of the hazards of toppling another Middle Eastern strongman. She pressed for a secret U.S. program that supplied arms to rebel militias, an effort never before confirmed.

Only after Gadhafi fell and what one U.S. diplomat called “the endorphins of revolution” faded did it become clear that Libya’s new leaders were unequal to the task of unifying the country, and that the elections Clinton and Obama pointed to as proof of success only deepened Libya’s divisions.

Now Libya, with a population smaller than that of Tennessee, poses an outsize security threat to the region and beyond, calling into question whether the intervention prevented a humanitarian catastrophe or merely helped create one of a different kind.

The looting of Gadhafi’s vast weapons arsenals during the intervention has fed the Syrian civil war, empowered terrorist and criminal groups from Nigeria to Sinai, and destabilized Mali, where Islamist militants stormed a Radisson hotel in November and killed 20 people.

A growing trade in humans has sent a quarter-million refugees north across the Mediterranean, with hundreds drowning en route. A civil war in Libya has left the country with two rival governments, cities in ruins and more than 4,000 dead.

Amid that fighting, the Islamic State has built its most important outpost on the Libyan shore, a redoubt to fall back upon as it is bombed in Syria and Iraq. With the Pentagon saying the Islamic State’s fast-growing force now numbers between 5,000 and 6,500 fighters, some of Obama’s top national security aides are pressing for a second U.S. military intervention in Libya. On Feb. 19, U.S. warplanes hunting a Tunisian militant bombed an Islamic State training camp in western Libya, killing at least 41 people.

“We had a dream,” said Jibril, who served as Libya’s first interim prime minister. “And to be honest with you, we had a golden opportunity to bring this country back to life. Unfortunately, that dream was shattered.”

On the campaign trail and in relentless congressional investigations, Republican critics have used a singular tragedy, the Sept. 11, 2012, terrorist attack on the U.S. diplomatic complex in Benghazi, which killed Ambassador J. Christopher Stevens and three other Americans, as a hammer against the former secretary of state. And while attempts to pin blame on Clinton have largely been frustrated, her rival for the Democratic presidential nomination, Sen. Bernie Sanders of Vermont, has seized on her role in the larger narrative of the Libyan intervention; during a recent debate, he said he feared that “Secretary Clinton is too much into regime change.”

Obama has called failing to do more in Libya his biggest foreign policy lesson. And Gérard Araud, the French ambassador to the United Nations during the revolution, is deeply troubled by the aftermath of the 2011 intervention: the Islamic State only “300 miles from Europe,” a refugee crisis that “is a human tragedy as well as a political one” and the destabilization of much of West Africa.

“You have to make a moral choice: a blood bath in Benghazi and keeping Gadhafi in power, or what is happening now,” Araud said. “It is a tough question, because now Western national interests are very much impacted by what is happening in Libya.”

Another Mideast War

It was late afternoon on March 15, 2011, and Araud had just left the office when his phone rang. It was his U.S. counterpart, Susan E. Rice, with a pointed message.

France and Britain were pushing hard for a Security Council vote on a resolution supporting a no-fly zone in Libya to prevent Gadhafi from slaughtering his opponents. Rice was calling to push back, in characteristically salty language.

“She says, and I quote, ‘You are not going to drag us into your shitty war,’” said Araud, now France’s ambassador in Washington. “She said, ‘We’ll be obliged to follow and support you, and we don’t want to.’ The conversation got tense. I answered, ‘France isn’t a U.S. subsidiary.’ It was the Obama policy at the time that they didn’t want a new Arab war.”

In the preceding weeks, a series of high-level meetings had grappled with the escalating rebellion, and some younger White House aides believed the president should join the international effort.

But a far more formidable lineup was outspoken against a U.S. commitment, including Vice President Joe Biden; Tom Donilon, the national security adviser; and Gates, the defense secretary, who did not want to divert U.S. air power or attention away from Afghanistan and Iraq. If the Europeans were so worried about Libya, they argued, let them take responsibility for its future.

“I think at one point I said, ‘Can I finish the two wars I’m already in before you guys go looking for a third one?’” Gates recalled. Gadhafi, he said, “was not a threat to us anywhere. He was a threat to his own people, and that was about it.”

Some senior intelligence officials had deep misgivings about what would happen if Gadhafi lost control. In recent years, the Libyan dictator had begun aiding the United States in its fight against al-Qaida in North Africa.

“He was a thug in a dangerous neighborhood,” said Michael T. Flynn, a retired Army lieutenant general who headed the Defense Intelligence Agency at the time. “But he was keeping order.”

Then there was Clinton. Early in Obama’s presidency, she had worked hard to win the trust of the man who had bested her in a tough primary campaign in 2008, and she sometimes showed anxiety about being cut out of his inner circle. (In one 2009 email, she fretted to aides: “I heard on the radio that there is a Cabinet mtg this am. Is there? Can I go?”)

Clinton had cultivated a close relationship with Gates. Both tended to be more hawkish than the president. They had raised concerns about how rapidly he wanted to withdraw troops from Afghanistan. More recently, they had argued that Obama should not be too hasty in dropping support for Hosni Mubarak, the embattled Egyptian leader, whom Clinton had known since her years as the first lady.

But they had lost out to the younger aides — “the backbenchers,” Gates called them, who he said argued that in the moral clash of the Arab Spring, “Mr. President, you’ve got to be on the right side of history.”

In Libya, Clinton had a new opportunity to support the historic change that had just swept out the leaders of its neighbors Egypt and Tunisia. And Libya seemed a tantalizingly easy case — with just 6 million people, no sectarian divide and plenty of oil.

But the debate was handicapped by sketchy intelligence. Top State Department officials were busy trying to evacuate the American Embassy, fearing that the Libyan leader might use diplomats as hostages. There was no inside information on whether, or on what scale, Gadhafi would carry out his threats.

“We, the U.S., did not have a particularly good handle on what was going on inside Libya,” said Derek Chollet, a State Department aide who moved to the National Security Council as the Libya debate began. U.S. officials were relying largely on news reports, he said.

Human Rights Watch would later count about 350 protesters killed before the intervention — not the thousands described in some media accounts. But inside the Obama administration, few doubted that Gadhafi would do what it took to remain in power.

“Of course, he would have lined up the tanks and just gone after folks,” said David H. Petraeus, the retired general and former CIA director.

Jake Sullivan, Clinton’s top foreign-policy aide at State and now in her campaign, said her view was that “we have to live in a world of risks.” In assessing the situation in Libya, he said, “she didn’t know for certain at the time, nor did any of us, what would happen — only that it passed a risk threshold that demanded that we look very hard at the response.”

So, after some initial doubts, Clinton diverged from the other senior members of the administration.

The comparison with Biden was revealing. For the vice president, according to Antony J. Blinken, then his national security adviser and now deputy secretary of state, the lesson of Iraq was crucial — “what Biden called not the day after, but the decade after.”

“What’s the plan?” Blinken continued. “There is going to be some kind of vacuum, and how’s it going to be filled, and what are we doing to fill it?” Former Secretary of State Colin Powell’s famous adage about Iraq — if “you break it, you own it” — loomed large.

More decisive for Clinton were two episodes from her husband’s presidency — the U.S. failure to prevent the Rwandan genocide in 1994, and the success, albeit belated, in bringing together an international military coalition to prevent greater bloodshed after 8,000 Muslims were massacred in Srebrenica during the Bosnian war.

“The thing about Rwanda that’s important is it showed the cost of inaction,” said James B. Steinberg, who served as Clinton’s deputy through July 2011. “But I think the reason Bosnia and Kosovo figured so importantly is they demonstrated there were ways of being effective and there were lessons of what worked and didn’t work.”

‘We Will Be Left Behind’

On the same March afternoon when Rice was telling her French colleague at the United Nations to back off, Obama and his security Cabinet were arrayed in the White House situation room. Speaking on the video screen from Cairo was Clinton, just arrived from Paris.

The day before, at lunch with President Nicolas Sarkozy of France, she “was tough, she was bullish” on the idea of intervention in Libya — the “perfect ally,” recalled Sarkozy’s senior diplomatic adviser, Jean-David Levitte.

But now Clinton did not directly push Obama to intervene in Libya. Nor did she make an impassioned moral case, according to several people in the room.

Instead, she described Jibril, the opposition leader, as impressive and reasonable. She conveyed her surprise that Arab leaders not only supported military action but, in some cases, were willing to participate. Mostly, though, she warned that the French and British would go ahead with airstrikes on their own, potentially requiring the United States to step in later if things went badly.

Dennis B. Ross, then a senior Middle East expert at the National Security Council, said he remembered listening to her and thinking, “If she’s advocating, she’s advocating in what I would describe as a fairly clever way.”

He recalled her saying: “You don’t see what the mood is here, and how this has a kind of momentum of its own. And we will be left behind, and we’ll be less capable of shaping this.”

Clinton’s account of a unified European-Arab front powerfully influenced Obama. “Because the president would never have done this thing on our own,” said Benjamin J. Rhodes, the deputy national security adviser.

Gates, among others, thought Clinton’s backing decisive. Obama later told him privately in the Oval Office, he said, that the Libya decision was “51-49.”

“I’ve always thought that Hillary’s support for the broader mission in Libya put the president on the 51 side of the line for a more aggressive approach,” Gates said. Had the secretaries of state and defense both opposed the war, he and others said, the president’s decision might have been politically impossible.

Having decided to act, Obama questioned military leaders about the effectiveness of a no-fly zone, the Europeans’ favored military response. When they told him that it could not prevent a massacre, Obama directed his staff to draft a new, tougher U.N. resolution.

Late that night, Araud, the French diplomat, was astonished to get a second call from Rice: The United States would not only support intervention, but wanted U.N. support for more than a no-fly zone. Araud said the turnabout had so shocked him and his British counterpart that they at first suspected a trick.

There remained only one real obstacle: Russia could block a Security Council resolution with a veto. Clinton had done her best to develop a relationship with Russia’s leader, Vladimir Putin, listening to his tales of tagging polar bears and tracking Siberian tigers.

“Her theory on Putin is, this is a person with some passions — if you get him going on those passions, your capacity to try and deal with him is improved,” one Clinton aide said.

But the relationship remained difficult, and the secretary of state sparred constantly with her Russian counterpart, Sergey V. Lavrov, who, Clinton wrote in her memoir “Hard Choices,” was initially “dead set against a no-fly zone.”

“We don’t want another war,” she told Lavrov, stressing that the mission was limited to protecting civilians.

“I take your point about not seeking another war,” she recalled him responding. “But that doesn’t mean that you won’t get one.”

In the end, Clinton would acknowledge that Gadhafi himself had helped win over the Russians, by giving a fiery speech just before the Security Council vote calling his opponents “the rats” and vowing to hunt them “house by house, alley by alley.”

On March 17, 10 members of the Security Council voted for a resolution authorizing “all necessary means” to protect Libyan civilians. Five countries, including Russia, abstained.

Two days later, Sarkozy met with Clinton and David Cameron, the British prime minister, at the Élysée Palace in Paris to discuss the next move. The French president emphasized that within a day or so, Gadhafi’s troops would be inside Benghazi, mingling with civilians, making it difficult or impossible to use air power against them.

Then he played his trump card. French fighter jets were already in the air, he said. But, he added, “this is a collective decision, and I will recall them if you want me to,” Levitte said. Sarkozy’s maneuver had abruptly pushed forward the timing of the operation, but for all of Clinton’s irritation, she was not prepared to object.

“I’m not going to be the one to recall the planes and create the massacre in Benghazi,” she grumbled to an aide. And the bombing began.

The Mission Shifts

Early on, Obama had declared that Gadhafi had lost his legitimacy and had to go. But the president was careful to point out that this was the administration’s political position, not its military objective.

“We are not going to use force to go beyond a well-defined goal, specifically the protection of civilians in Libya,” he said. Clinton echoed that five days after the Security Council resolution was adopted. “There is nothing in there about getting rid of anybody,” she told ABC News.

The president directed the Pentagon to use its unique military capabilities to stop the feared massacre and, within 10 days, turn the operation over to European and Arab allies. An unnamed aide described this approach as “leading from behind,” handing the president’s Republican opponents an enduring talking point. But Obama was adamant that Libya would not become another protracted U.S. war.

In fact, his limited goal was achieved far faster than planned. “We basically destroyed Gadhafi’s air defenses and stopped the advance of his forces within three days,” recalled Rhodes, the deputy national security adviser.

But the mission quickly evolved from protecting civilians in Benghazi to protecting civilians wherever they were. As the rebellion swelled and bystanders became combatants, the endgame became ever more murky. The United States and its allies were increasingly drawn to one side of the fighting, without extended debate over what that shift portended.

“I can’t recall any specific decision that said, ‘Well, let’s just take him out,’” Gates said. Publicly, he said, “the fiction was maintained” that the goal was limited to disabling Gadhafi’s command and control. In fact, the former defense secretary said, “I don’t think there was a day that passed that people didn’t hope he would be in one of those command and control centers.”

Two of Clinton’s top Libya advisers said in interviews that they had harbored misgivings about the intervention precisely because of fears that the coalition would not be able to stop short of regime change, with no ability to manage the aftermath.

One was Gordon, the assistant secretary. The other was Jeremy Shapiro, who handled Libya on Clinton’s policy planning staff.

Shapiro said he had expressed his concerns to Clinton’s top policy aide, Sullivan. “Once you get into a fight where we basically say, ‘We have to stop a madman from killing tens of thousands of people in his own country,’ how do you stop?” Shapiro said.

“Ultimately the logic becomes, Jesus, the Gadhafi regime is a real threat to civilians,” he added. “It required nothing to escalate to that. It would have required an amazing force of will not to.”

Arming the Rebels

When Jibril and his Libyan entourage showed up in Rome in May to meet with Clinton, they expected a 10-minute check-in. Instead, they talked for nearly an hour.

The opposition leaders had already given her a white paper setting out a spectacular future: Political parties would compete in open elections, a free news media would hold leaders accountable and women’s rights would be respected.

In retrospect, Jibril acknowledged in an interview, it was a “utopian ideal” quite detached from Libyan reality. But Clinton had been enthusiastic, according to those in attendance, and now she wanted to talk in greater depth about how to turn the vision into reality.

“She said, and I remember this, ‘Let us brainstorm about Libya,’” said Mahmud Shammam, the rebel council’s chief spokesman.

The opposition leaders wanted something more immediate. They wanted weapons.

Despite hundreds of coalition airstrikes, the fighting was at a stalemate. Every time the rebels gained some ground, government forces retook it. The rebels seemed unable to get past Brega, an oil port on the way to Tripoli, and they hoped more sophisticated weapons from the Americans would tip the balance.

The secretary of state heard them out. She “was very patient, very charming,” Shammam said. “Always had a smile.” In the end, though, she demurred.

But back in Washington, where a low-grade panic over the stalled fighting was setting in, Clinton pressed the rebels’ case, according to three senior White House officials and two State Department officials involved in the secret debate.

The U.S. military involvement that Obama had hoped to curtail after 10 days had dragged on for months, and political support was waning. Some members of Congress were outraged over the administration’s failure to seek approval after 60 days, as the War Powers Act seemed to require.

Onetime advocates of the intervention, including Slaughter, the secretary’s former policy planning director, had grown disillusioned over the rebels’ human-rights abuses.

“We did not try to protect civilians on Gadhafi’s side,” said Slaughter, who at the time called for a deal in which Gadhafi would have turned over power to one of his sons.

The international coalition that Clinton had stitched together was also unraveling. Russia accused the United States and its allies of a bait-and-switch, and the Arab League called for a cease-fire and settlement.

“Regime change — that was not our business at all,” Amr Moussa, who headed the organization at the time, said in an interview.

“There was a moment, around about June or July,” recalled Shapiro, the State Department’s Libya policy adviser, “when the situation on the ground seemed to settle into a stalemate and we weren’t sure we were winning, or at least winning quickly enough.”

Moreover, the United States’ strategy of letting other countries arm the opposition was backfiring, creating a regional power imbalance that could come back to haunt Libya if the rebels did win.

Throughout the spring, the administration had effectively turned a blind eye as Qatar and the United Arab Emirates supplied the rebels with lethal assistance, according to Gates and others. But Clinton had grown increasingly concerned that Qatar, in particular, was sending arms only to certain rebel factions: militias from the city of Misrata and select Islamist brigades.

She could hardly tell Qatar to stand down if the United States was unwilling to step in with lethal assistance of its own, one State Department aide said, “because their answer would be, ‘Well, those guys need help — you’re not doing it.’” Her view, often relayed to her staff, was that to have influence with the fractious opposition and Arab allies, you had to have “skin in the game,” Ross said.

Former President Bill Clinton had publicly noted in April 2011 that the United States should “not rule out” arming the opposition, and in emails with Sullivan, her policy adviser, Hillary Clinton discussed using private contractors to do just that. Ross, speaking generally, said she had frequently consulted her husband: “I’d say, ‘Here’s what I think we should do.’ She’d say, ‘That’s what Bill said, too.’”

Now Hillary Clinton took what one top adviser called “the activist side” of the debate over whether to counter Qatar by arming more secular fighters.

“If you didn’t,” Ross recalled her arguing, “whatever happened, your options would shrink, your influence would shrink, therefore your ability to affect anything there would also shrink.”

But other senior officials were wary. NATO’s supreme allied commander, Adm. James G. Stavridis, had told Congress of “flickers” of al-Qaida within the opposition. Donilon, Obama’s national security adviser, argued that the administration could not ensure that weapons intended for “the so-called good guys,” as one State Department official put it, did not fall into the hands of Islamist extremists.

In fact, there was reason to worry. Jibril himself described in an interview how a French shipment of missiles and machine guns had gone awry. At a June meeting, Sarkozy had agreed to “ask our Arab friends” to supply the Transitional National Council with the weapons, Jibril said. But, he said, the acting defense minister diverted them to a militia led by Abdel Hakim Belhaj, a militant Islamist who had once been held in a secret prison by the CIA.

Clinton understood the hazards, but also weighed the costs of not acting, aides said. They described her as comfortable with feeling her way through a problem without being certain of the outcome.

Obama ultimately took her side, according to the administration officials who described the debate. After he signed a secret document called a presidential finding, approving a covert operation, a list of approved weaponry was drawn up. The shipments arranged by the United States and other Western countries generally arrived through the port of Benghazi and airports in eastern Libya, a Libyan rebel commander said.

“Humvees, counterbattery radar, TOW missiles was the highest end we talked about,” one State Department official recalled. “We were definitely giving them lethal assistance. We’d crossed that line.”

Prompted in part by the decision to arm the rebels, the State Department recognized the Transitional National Council as the “legitimate governing authority for Libya.” Clinton announced the decision on July 15 in Istanbul.

“That very day, our troops had started to get inside Brega,” Shammam recalled. “We told that to Mrs. Clinton, and she said — I remember her smiling — ‘Good! This is the only language that Gadhafi is understanding.’”

‘Days Are Numbered’

One month later, Clinton appeared at the National Defense University with Leon E. Panetta, who had recently replaced Gates as defense secretary. She hailed the intervention as a case study in “smart power.”

“For the first time we have a NATO-Arab alliance taking action, you’ve got Arab countries who are running strike actions,” she said. “This is exactly the kind of world that I want to see where it’s not just the United States and everybody is standing on the sidelines while we bear the cost, while we bear the sacrifice.”

Panetta spoke of a “sense that Gadhafi’s days are numbered.”

Six days later, on Aug. 22, the cumulative efforts of the international coalition bore fruit when exuberant rebels stormed the Gadhafi compound in Tripoli. The dictator was still at large, but his reign was over.

Clinton’s old friend and political adviser, Sidney Blumenthal, who regularly emailed her political advice and vaguely sourced intelligence reports on Libya, urged her to capitalize on the dictator’s fall.

“Brava!” Blumenthal exclaimed. As always, he was thinking about Clinton’s presidential ambitions. “You must go on camera. You must establish yourself in the historical record at this moment.” She should be sure to use the phrase “successful strategy,” he wrote. “You are vindicated.”