Once, Libya seemed set to stand as a triumphant test case of Hillary Clinton's vision on the smart application of U.S. power abroad.

But the U.S.-led operation to avert genocide, which eventually led to the ouster of longtime dictator Moammar Gadhafi, left a political vacuum. It has since been exploited by groups like al Qaeda and ISIS that have identified lawless territory ripe for exploitation and a possible beachhead to launch terrorist attacks on Europe. And it has spawned a humanitarian crisis that has seen boatloads of refugees risk dangerous passage across the Mediterranean.

The former secretary of state's key role in building the Western and Arab coalition responsible for enforcing a no-fly zone over Syria in 2011 is likely to come under scrutiny again, as Clinton appears before the House Select Committee on Benghazi on Thursday in a pivotal moment for her 2016 presidential campaign.

The hearing is primarily designed for Republicans to grill the Democratic front-runner over the attack on U.S. consular buildings in the Libyan city on September 11, 2012, in which U.S. Ambassador Christopher Stevens and three other Americans were killed.

But Libya's lingering agony is also creating an opening for Republican presidential candidates to question Clinton's strategic acumen and to undermine her diplomatic credentials, which will be at the center of her pitch that only she has the global experience needed to be president in a turbulent time.

Gathering questions over Libya also point to one of the central complications of Clinton's campaign for the Democratic nomination: The fact that she must own a record at the State Department that lacks clear-cut diplomatic triumphs. In addition, she'll have to answer for misfires in the Obama administration's wider foreign policy as GOP candidates who have not faced the same tough choices can nitpick her record with the advantage of hindsight.

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Republicans have yet to prove that she was personally negligent or to convince voters that she's not fit for higher office because of the controversy. But the Libya episode still offers potential for candidates keen to undermine Clinton's foreign policy credentials.

Kentucky Sen. Rand Paul, a GOP presidential candidate, has called Libya a "jihadist wonderland" and Hillary Clinton a "war hawk" in questioning the mission, which he is using to flesh out a wider critique that American military interventions in the Middle East have backfired badly.

"Somebody needs to ask Hillary Clinton, was it a good idea to topple Gadhafi in Libya? I think it's a disaster. Libya is a failed state. Someone ought to pay and Hillary Clinton needs to answer questions about it," Paul said at an Iowa Republican Party Lincoln Dinner in Des Moines on May 16.

Another GOP presidential candidate, Florida Sen. Marco Rubio, said in an interview with Charlie Rose of PBS in May that "the result in Libya was a protracted conflict that killed people, destroyed infrastructure and left behind the conditions for the rise of multiple militias who refused to lay down their arms."

Clinton defends her role in Libya

But Clinton is unapologetic about her role in Libya. In fact, she's holding up the limited U.S. action there as an ideal template for American interventions abroad in which allies do the front line fighting.

"We had a murderous dictator, Gadhafi, who had American blood on his hands, as I'm sure you remember, threatening to massacre large numbers of the Libyan people," Clinton said at the CNN Democratic debate in Las Vegas last week.

"We had our closest allies in Europe burning up the phone lines begging us to help them try to prevent what they saw as a mass genocide, in their words. And we had the Arabs standing by our side saying, 'We want you to help us deal with Gadhafi.'

"Our response, which I think was smart power at its best, is that the United States will not lead this. We will provide essential, unique capabilities that we have, but the Europeans and the Arabs had to be first over the line. We did not put one single American soldier on the ground in Libya."

Clinton allies stress that Obama's national security aides back in 2011 feared that that Gadhafi, who had threatened to hunt down opponents like rats, was poised to unleash a massacre in Benghazi as his forces advanced on rebels crowded in the city in March 2011.

It is clear that Clinton and other top administration aides perceived an agonizing dilemma: Should they take action to avert human carnage or stand by and be accused of abetting genocide?

"Imagine we were sitting here and Benghazi had been overrun, a city of 700,000 people, and tens of thousands of people had been slaughtered, hundreds of thousands had fled," Clinton said on ABC's "This Week" in March 2011. "The cries would be, 'Why did the United States not do anything?' "

Clinton might have been thinking about the recriminations that flew when her husband Bill Clinton's administration failed to thwart a genocide in Rwanda in 1994. The former president has since said his decision not to intervene was one of his biggest regrets.

Initially, the operation was billed as a purely humanitarian effort to create a no-fly zone to thwart Gadhafi's forces, which was endorsed by a U.N. Security Council resolution.

But air attacks on Libyan forces designed to make the skies safe for Western aircraft helped tip the balance of the battlefield toward the rebels and left critics of the war, such as Russia, convinced that an international coalition under the umbrella of NATO had always been bent on regime change.

In the end, Gadhafi was indeed toppled, capping a four-decades-long legacy steeped in blood, including violent repression at home and accusations that he was behind the bombing of a U.S. airliner over Lockerbie, Scotland, in 1988.

Initially, the Libya intervention seemed set to go down as one of the Obama administration's triumphs.

"We came, we saw, he died," Clinton joked with a television reporter in the wake of the death of the fugitive Gadhafi at the hands of a mob, days after she visited Tripoli in support of Libya's transitional government.

But when Gadhafi fell, he left a land of eviscerated political institutions ill-prepared for its sudden freedom.

Libya's painful breakdown

Torn by tribalism, ruled by two rival parliaments and with hundreds of civilians killed during the subsequent civil war, Libya is suffering a complete breakdown of political authority, which has spurred the rise of a host of terror groups. The unrest has also spawned a huge refugee crisis as desperate migrants flee across the Mediterranean on overcrowded boats.

And where chaos and anarchy reign, terror groups often see opportunity.

ISIS is seeking to expand its franchise in Libya and to grab a foothold from which it could export terror across the Mediterranean to Europe. The surf literally ran with blood earlier this year when ISIS beheaded 21 Egyptian Christians in a show of brutality on a Libyan beach.

While some experts blame Libya's interim leaders themselves for not doing enough to stem the terror tide and keep the country together after the war, even Obama admits that he made mistakes in Libya. In an interview with The New York Times last year, he defended the intervention but said he should have asked, "Do we have an answer for the day after?"

For Clinton's part, she seemed to indicate in her book "Hard Choices" that she knew all along that Libya would face a tough road.

"I was worried that the challenges ahead would prove overwhelming for even the most well-meaning transitional leaders," Clinton wrote.

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"If the new government could consolidate its authority, provide security, use oil revenues to rebuild, disarm the militias, and keep extremists out, then Libya would have a fighting chance at building a stable democracy.

"If not, then the country would face very difficult challenges translating the hopes of a revolution into a free, secure, and prosperous future. And, as we soon learned, not only Libyans would suffer if they failed."

Clinton has little choice but to own what happened in Libya. An email to Clinton in April 2012 from her former top adviser Jake Sullivan, released earlier this year, appears to show that initially her aides were keen to trumpet her role in the intervention and saw it as legacy-enhancing.

"HRC has been a critical voice on Libya in administration deliberations, at NATO, and in contact group meetings -- as well as the public face of the U.S. effort in Libya. She was instrumental in securing the authorization, building the coalition, and tightening the noose around Qadhafi and his regime," Sullivan wrote.

Former Defense Secretary Robert Gates also describes her pivotal role in the decision making in his memoir.

Gates said the intervention, which he initially opposed, split the administration down the middle, with heavy hitters such as Vice President Joe Biden and national security adviser Tom Donilon also against.

On the other side were U.N. Ambassador Susan Rice and National Security Council staffers including Ben Rhodes and Samantha Power.

"In the final phase of the internal debate, Hillary threw her considerable clout behind Rice, Rhodes and Power," Gates wrote.

A political risk, then and now

Clinton's prominent role in the decision making on Libya was a political risk at the time, and it now threatens to return to haunt her four years on.

"We haven't gotten the full story yet, but from everything we do know, it appears that without her advocacy for this intervention, it wouldn't have happened," said Alan Kuperman of the Lyndon B. Johnson school of Public Affairs at the University of Texas, who has studied the causes and outcome of the Libya operation, in an interview earlier this year.

Some critics now question whether the administration, presumably working from intelligence provided by rebels, miscalculated on Gadhafi's intentions. And they say that the administration did not do enough to consider the consequences of an operation that ended up toppling Gadhafi.

"If you were going to break this place, it was going to require enormous resources to keep it together," Kuperman said. "It would have required an occupation force, and it was clear that the U.S. did not have the stomach for that."

He concluded: "Did she screw up? Yes, she screwed up."

But one U.S. official involved with the planning of the Libya engagement defended Clinton's position.

"I think that it is tempting always in hindsight to say Libya would be better somehow if we had not intervened," said Derek Chollet, a close national security adviser to Clinton and Obama at the time of the Libya operation told CNN in an interview earlier this year.

"I think that is a highly dubious proposition. It is one where, had we not intervened, the conversation we would be having is thousands and thousands of people died and Libya looks like Syria."

But it is always hard in political campaigns to prove that but for a specific course of action, things could have turned out worse than they actually did. And Clinton's opponents, especially Republicans who want to dismantle her credentials as a potential commander in chief, are already signaling that Libya will be a significant political battleground in 2016.