At dawn on March 19, 2011, artillery rained down on the Libyan rebel capital of Benghazi. Col. Muammar Qaddafi appeared poised to make good on his threats to exterminate the "rats" seeking his ouster. Thousands of families fled.

Then French warplanes sprang into action, soon to be joined by US, British, and other NATO forces. Qaddafi's armored column was reduced to scrap metal, paving the way for his eventual overthrow.

Now, a similar scenario is taking place in Syrian cities like Homs, where withering artillery barrages that began Feb. 3 have killed at least 100 civilians – some say the number could be in the several hundreds – and flooded YouTube and social networking sites with horrific images. At least 5,400 Syrians have been killed in the 11-month uprising. But the prospects for action like the NATO intervention in Libya are virtually nil.

The barrage on Homs, a center for opposition to the continued rule of Syrian President Bashar al-Assad, eased on Saturday as the Arab League held an emergency meeting in Cairo on the war there. The group called on the UN Security Council to send peace-keepers to Syria to end the bloodshed. The opposition in Syria has been braying for international military assistance for weeks. But it remains highly unlikely that they will get the direct intervention they're asking for.

Supporters of liberal intervention – the notion that international powers should override national sovereignty to protect civilians and improve societies – had crowed that Libya was a model for the 21st century, a further pillar supporting the "responsibility to protect." .

But Syria's war, longer and bloodier than when NATO intervened in Libya's, is not stirring anyone to action. There are differences between Libya and Syria in alliances, terrain, and military capability. International action would be longer, costlier, and would probably require an invasion to be fully effective. And at any rate, Russia and China, angry over how the UN's role in ousting Col. Qaddafi played out, have vowed to veto attempts at action in Syria.

Strengths and weaknesses

While the Libyan intervention was sold on moral grounds, the fact that it was a comparatively easy mission was also influential. Qaddafi had a weak military, paltry air defenses, and no powerful friends left internationally. Libya's vast desert landscape, with population centers clustered in a thin band along the Mediterranean coast, rendered it almost tailor-made for intervention from the air.

By contrast, Syria's population density is almost 30 times greater, which increases the risk of civilian casualties. The Army is five times larger than the former Libyan Army and much better equipped. And Syria's air-defense network is sufficiently large to pose a challenge to Western planes. "In Libya, we could accomplish a lot of value with very little at stake," says Joshua Foust, a fellow at the American Security Project in Washington, D.C. "Syria has an advanced antiaircraft system that would take a lot of work to get rid of. You'd have to engage in a lot of destruction and commit a lot of time and a lot of money to get to the point where you could accomplish something."

On top of the technical military problems is the charged regional context. The Libyan intervention took place in a calmer neighborhood. Syria – Iran's best friend and a key sponsor of the militant group Hezbollah in Lebanon – is already becoming an arena for proxy battles between regional forces, says Aram Nerguizian. He is a military analyst with the Center for Strategic and International Studies in Washington and author of a December report on the risks of military intervention in Syria. "Anything that would involve direct Western intervention would be deeply destabilizing at the regional level," he says.

Syria's social landscape is more complicated than Libya's as well. Like Iraq, where millions of Sunni Arabs and Christians were driven out during the sectarian civil war that followed Saddam Hussein's ouster, Syria is governed by a privileged minority: the Alawites. The Muslim sect makes up about 12 percent of the population. Many Alawites believe winning the war is a matter of survival and fear the reprisals that would follow a victory by the Sunni majority. Iraq and Lebanon (which endured a 15-year civil war) serve as reminders of the horrors of sectarian conflict.