The US State Department designated the Jabhat al-Nusra militia fighting Bashar al-Assad's government in Syria a foreign terrorist organization today.

The speed with which the US government moved to designate a fairly new group that has never attacked US interests and is engaged in fighting a regime that successive administrations have demonized is evidence of the strange bedfellows and overlapping agendas that make the Syrian civil war so explosive.

The State Department says Jabhat al-Nusra (or the "Nusra Front") is essentially a wing of Al Qaeda in Iraq, the jihadi group that flourished in Anbar Province after the US invaded to topple the Baathist regime of secular dictator Saddam Hussein. During the Iraq war, Sunni Arab tribesmen living along the Euphrates in eastern Syria flocked to fight with the friends and relatives in the towns along the Euphrates river in Anbar Province.

The terrain, both actual and human, is similar on both sides of that border, and the rat lines that kept foreign fighters and money flowing into Iraq from Syria work just as well in reverse. Now, the jihadis who fought and largely lost against the Shiite political ascendancy in Iraq are flocking to eastern Syria to repay a debt of gratitude in a battle that looks more likely to succeed every day.

The Nusra Front has gone from victory to victory in eastern Syria and has shown signs of both significant funding and greater military prowess than the average citizens' militia, with veterans of fighting in Iraq, Afghanistan, and Libya among its numbers.

The US of course aided the fight in Libya to bring down Muammar Qaddafi. In Afghanistan and Iraq, the chance to fight and kill Americans was the major drawing card.

In Iraq, the US toppled a Baathist dictatorship dominated by Sunni Arabs, opening the door for the political dominance of Iraq's Shiite Arab majority and the fury of the country's Sunni jihadis. In Syria, a Baathist regime dominated by the tiny Alawite sect (a long-ago offshoot of Shiite Islam) risks being brought down by the Sunni majority. Iraq's Shiite Prime Minister Nouri al-Maliki is in the odd position of now rooting for a Baathist regime to survive, frightened that a religiously inspired Sunni regime may replace Assad and potentially destabilize parts of his country from Haditha in Anbar's far west to the northern city of Mosul.

For the US, the situation is more complicated still. The Obama administration appears eager for Assad to fall, but is also afraid of what might replace him, not least because of Syria's chemical weapons stockpile. If the regime collapses, the aftermath is sure to be chaotic, much as it was in Libya, where arms stores were looted throughout the country. The presence of VX and sarin nerve gas, and the fear of Al Qaeda aligned militants getting their hands on it, has the US considering sending in troops to secure the weapons.

That's the context in which today's designation was made – part of an overall effort to shape the Syrian opposition to US liking, and hopefully have influence in the political outcome if and when Assad's regime collapses. But while the US has been trying to find a government or leadership in waiting among Syrian exiles, Nusra has been going from strength to strength. Aaron Zelin, who tracks jihadi groups at the Washington Institute for Near East Policy, notes in a recent piece for Foreign Policy that 20 out of the 48 "martyrdom" notices posted on Al Qaeda forums for the Syria war were made by people claiming to be members of Nusra.

Zelin writes that it's highly unusual for the US to designate as a terrorist group anyone who hasn't attempted an attack on the US. In fact, the US only designated the Haqqani Network in Afghanistan, which had been involved in attacks on US troops there for over a decade, this September.

His guess as to why the US took such an unusual step?

The U.S. administration, in designating Jabhat al-Nusra, is likely to argue that the group is an outgrowth of the Islamic State of Iraq (ISI). While there is not much open-source evidence of this, classified material may offer proof -- and there is certainly circumstantial evidence that Jabhat al-Nusra operates as a branch of the ISI. Getting Syria's rebels to disavow Jabhat al-Nusra may not be an easy task, however. As in Iraq, jihadists have been some of the most effective and audacious fighters against the Assad regime, garnering respect from other rebel groups in the process. Jabhat al-Nusra seems to have learned from the mistakes of al Qaeda in Iraq: It has not attacked civilians randomly, nor has it shown wanton disregard for human life by publicizing videos showing the beheading of its enemies. Even if its views are extreme, it is getting the benefit of the doubt from other insurgents due to its prowess on the battlefield.

Will it hurt the group's support inside Syria? It's hard to see how. The US hasn't formally explained its logic yet, but it's hard to see how that will matter either. The rebellion against Assad has raged for almost two years now and the country's fighters are eager for victory, and revenge. The US has done little to militarily assist the rebellion, and fighters have been happy to take support where they can get it.

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Most of the money or weapons flowing into the country for rebels has come from Gulf states like Saudi Arabia and Qatar and some of that support, of course, has ended up in the hands of Islamist militias like Nusra.

Usually the US doesn't like support flowing to its designated terrorist organizations, and leans on countries like Saudi Arabia to cut off support. But in this case, a doctrinaire enforcement of its will could look like helping Assad (who has insisted everyone fighting his government is a terrorist since long before Nusra even existed).