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For interests on both sides of Syria’s civil war, this has been the week to increase the pressure. Hezbollah sent reinforcements to the troops of President Bashar al-Assad, and Russia reiterated its intention to furnish the regime with weapons. At the same time, Republican Sen. John McCain secretly visited rebels and promised to push the Obama administration to arm the retreating forces. The European Union allowed its weapons embargo to lapse as nations such as Britain and France appear increasingly eager to aid the opposition fighters.

But amid the burst in outside engagement, one influential group seems noticeably silent. The liberal hawks, a cast of prominent left-leaning intellectuals, played high-profile roles in advocating for American military intervention on foreign soil — whether for regime change or to prevent humanitarian disasters. They pressured President Bill Clinton to intervene in Bosnia, provided intellectual cover on the left for President George W. Bush’s war in Iraq and urged President Obama to engage in Libya. But even as the body count edges toward 100,000 in Syria and reports of apparent chemical-weapons use by Assad, liberal advocates for interceding have been rare, spooked perhaps by the traumatic experiences in Iraq and Afghanistan and the clear reluctance of a Democratic president to get mired in the Middle East. Call them Syria’s mourning doves.

“Everybody has their own ghosts to deal with,” said Vali Nasr, a former Obama administration official and leading proponent of intervention in Syrian. “But those people understand that what is going on in Syria cannot go on indefinitely.”

In their absence, the military-intervention-will-only-make-things-worse school of foreign policy subscribed to by key national security figures in the West Wing continues to hold sway.

“The reason that the president is being very discerning about how we react to the situation in Syria has very little to do with me,” said Denis McDonough, Obama’s chief of staff and longtime foreign policy adviser. “And very much to do with the president’s humility in recognizing the challenges of intervention in this region of the world, which is shown, I think in stark relief, with the situation in its neighbor — namely, Iraq.”

A look at the Syrian uprising nearly two years later. Thousands of Syrians have died and President Bashar al-Assad remains in power, despite numerous calls by the international community for him to step down.

The administration has maintained its advocacy for a political solution to remove Assad and is hoping to make progress at a peace conference next month.

In the face of administration reluctance to act, Nasr, the dean of the School of Advanced International Studies at Johns Hopkins and author of “The Dispensable Nation: American Foreign Policy in Retreat,” is spearheading the effort to mobilize the liberal hawks. Every day “I have more company,” he insisted, adding that he frequently receives encouragement in “private conversations.”

The advocates for intervention argue that there are strong moral, geopolitical and national security reasons to intervene and that although there is no good option in Syria, doing nothing is the worst option of all. They’d like to see the destruction of Assad’s aircraft, heavy weapons and assets on the ground and have called for the creation of no-fly zones and the arming of rebels, as well as a naval blockade to prevent Syria from exporting oil. They also believe American firepower could create havens in rebel-controlled territory, giving a moderate opposition the chance to govern and to weaken extremists while easing the burden of refugees, and preventing further sectarian spillover into neighboring states.

They worry that Obama is sending the message to dictators that brutality will go unchecked and that he is ceding the battlefield to the United States’ more strategic enemies, including Iran. The advocates worry that Obama’s blurring of red lines over the use of chemical weapons weakened American credibility and moral authority and reduced any chance for a diplomatic solution.

Those who oppose armed intervention fear that U.S. involvement in Syria would only worsen the situation and fuel the kind of sectarian fury that was unleashed in Iraq.

They also believe that the administration has been wise to avoid ownership of the problem if it is not willing to make a long-term nation-building commitment.

The few prominent liberal hawks have taken their case to high-profile platforms. Bill Keller, a former editor of the New York Times, recently acknowledged his wariness but added that “in Syria, I fear prudence has become fatalism, and our caution has been the father of missed opportunities, diminished credibility and enlarged tragedy.” He was immediately attacked with echoes of the “Bush’s Useful Idiots” critique. Leon Wieseltier has incessantly demanded action from his perch at the New Republic. And Anne-Marie Slaughter, a former top State Department official who is president of the New America Foundation, has been consistently outspoken in favor of intervention.

Slaughter said she wants to hear more from the intellectuals who joined her in urging intervention in Kosovo, Rwanda and, most recently, Libya. “The place to look, I think, is not 10 years ago [in Iraq], it’s Libya,” she said. “Where’s the Libya coalition?” She blamed the Obama administration inaction in Syria for creating a climate of “despairing futility” that rendered her former allies moot.

Other advocates of intervention have found some absentees particularly noteworthy.

“I am not the first or the only person who has wanted to know the views of the American writer who has done more than anyone else in recent years to emphasize the humanitarian responsibility of democratic countries in the face of giant massacres — namely, Samantha Power,” Paul Berman, an essayist and author who has been a consistent voice for intervention, wrote in an e-mail.

Power, who declined to comment, is being vetted for the position of undersecretary for civilian security, democracy and human rights at the State Department, according to several sources. She is said to be acting behind the scenes to build support for intervention in Syria. But given Obama’s apparent lack of interest — he told the New Republic that a humanitarian crisis in Syria alone does not justify U.S. military involvement — it is not clear whether having Power in the administration is as useful as having her as a clear voice outside it.

Other left-leaning supporters of previous interventions haven’t exactly been taking up the slack.

In the New York Times, Thomas Friedman, who supported the 2003 invasion of Iraq, wrote, “President Obama is right to be cautious about getting burned in Damascus” and warned that only a lengthy, expensive and total commitment of troops could end the civil war.

George Packer, one of the most compelling voices for humanitarian intervention in Iraq and elsewhere, argued in the New Yorker that as much as Obama may want to pivot toward the Asia-Pacific region, the Middle East requires his prolonged engagement. Since then, Packer has remained mostly silent, saying that Syria is not his area of expertise.

In an e-mail, Michael Ignatieff, a Harvard professor and author whose writing is said to be influential in the administration, wrote that he and others “have been saying something must be done and something can be done.” But fellow advocates of intervention said they wished Ignatieff would say such things in more prominent arenas so that the president might hear.

Even the former liberal hawks who have changed their minds have noticed that many of their one-time compatriots seem mum.

“However one feels about these issues, as a public intellectual you owe the public your best thoughts on it,” said Fareed Zakaria, who said the historical lessons of Iraq led him to oppose intervention in Syria. “I’ve tried to be as honest and open about my reservations about it.” He suggested that perhaps his fellow liberal hawks have been “somewhat chastened by the Iraq experience and relatedly how Afghanistan has turned out.”

The development has pleased traditional foreign policy realists, who applaud Obama’s reluctance to be drawn into the conflict.

“If you have a constant drumbeat in the op-ed columns and on the talk shows and inside the journals of opinion,” said Stephen Walt, a Harvard professor and leading realist thinker, “there are going to be some people within the administration who get influenced by it.”

Top administration officials agreed. “There is a very vibrant debate going on inside the administration,” McDonough said. “What is said on the outside is influential and obviously heard in that debate.” He added, though, that “the pressure about the situation in Syria comes from the threat to our interests and the unbelievable humanitarian disaster that’s unfolding.”

The one thing that both sides seem to agree on is that Obama, who in no small part owes his presidency to resisting the arguments of the liberal interventionists for war in Iraq, seems intent not to act or provide much space for the thinkers to aggregate.

Perhaps in the hopes of speaking the realist foreign policy language prevailing in the Oval Office, Nasr has framed the intervention as a national security, rather than humanitarian, imperative. But the intervention advocates who were so articulate in the past, he acknowledged, seem lost.

“The intellectuals are groping,” Nasr said. “They know where they were before is not probably right, but you don’t have a clarity of what we are going to do as a country.”

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