Some details and implications:
The attacking units apparently belong to the all-Kurdish YPG (People's Protection Units) and the SDF (Syrian Democratic Forces), a mostly Kurdish militia with a sprinkling of Arab recruits. The SDF is simply an unofficial asset of the PYD ( Syrian Kurdish Democratic Union Party), a Syrian Kurdish party fighting and holding territory in Northern Syria. The PYD and its YPG units are to all intents and purposes an arm of the Turkish PKK, one of the two main Kurdish insurgent groups in Iraq, Syria and Turkey itself.
The PYD has for over a year been a major recipient of US military support, including close air support, most notably in Kobane. Its territory is centered in Hassakeh province. It has had minor turf disputes with Assad, diligently inflated by pro-Kurdish propagandists. It has also clashed the Syrian rebels, including the FSA. More important, it has had at the very least a long-standing modus vivendi with the régime. More recently, collaboration with Assad through his Baath party has almost come out into the open:
LCC [a rebel information agency] was informed that Hilal al-Hilal, Assistant Regional Secretary in Baath Party, visited Hasaka city (northeast of Syria) in Feb 02 and met with Kurdish Units accompanied by party leaders and security personnel of the regime in the city. LCC sources said that the attendees discussed coordination between both sides on city administration. Al-Hilal promised the Kurdish Units in the meeting with ammos and weapons support to fight against ISIS in the southern suburbs. The source mentioned also that regime’s forces is about to form a new military troop of “Volunteer Brigades”, and supervised by Hezbollah to support the Kurdish Units in their war with ISIS.Like every announcement of Western support for the PYD, this report portrays régime-Kurdish collaboration as part of the fight against ISIS. However Hezbollah is, of course, a Iranian-backed militia that has been the most prominent among Assad's non-Syrian support troops. It virtually never fights ISIS but only the rebels.
From these reports it follows that the US is now underwriting a Kurdish movement which attacks, not only ISIS, but the rebels. In particular it is now part of a major joint assault on crucial positions of the FSA (Free Syrian Army), the very group the US purports to support. This assault is backed by Russian air strikes and complemented by simultaneous régime attacks. The Kurdish attackers belong to an extension of the very same organizations whose main support, up to now, has been the United States. Wherever Obama renders these organizations stronger, including all advances against ISIS, he frees up resources for attacks on the FSA. He does so, not in a small way, but to a crucial extent. So while the US condemns Assad and Russia, at the same time it backs attacks on the opposition to the régime - not just any part of the opposition, but the part which the US has 'vetted' as free from extremist leanings.
This is not entirely surprising. It has long been clear that, at the end of the day, the US prefers the atrocious reality of Assad to the possibility that a rebel group, any rebel group, comes to power. That's because the US doesn't trust even 'vetted' rebel groups to remain free of Islamist taint.
Perhaps more surprising is the curtain of silence drawn across the scene by almost every source of information allegedly disgusted with Assad and sympathetic to at least some of the rebels. Even genuine experts on the situation don't say plainly that the Kurds are attacking the FSA with régime and Russian support. On the contrary they express themselves so obliquely the average reader could never know what's going on:
YPG forces in Efrin appear to be receiving Russian air support, particularly near Azaz, a key city currently occupied by elements of the Turkish-backed anti-Assad insurgency. Open source airstrike data suggests that the SDF could seize Manbij with US backing, while the Assad regime moves north from Aleppo to Al Bab. The YPG, in turn, could then cut a deal with the regime to travel through regime held territory to Efrin.Thus the FSA becomes 'elements of the Turkish-backed anti-Assad insurgency', which could mean extremists. There is not even a clear mention of any Kurdish offensive - just the support they 'appear' to be receiving which 'could' enable them to seize some locations. But passages like this, buried in peripheral information sites, are a model of forthrightness compared to what appears, or rather doesn't appear, elsewhere. No major newspaper speaks of the Kurdish offensive - surely one of the biggest developments in the entire five-year conflict. None of the prominent supposedly anti-Assad analysts mention it. Expert military observers who seem to know the movements of every tank in Syria say nothing. Almost invariably, Kurdish attacks on the rebels are studiously ignored in favour of imprecations against Russia and Assad.
We seem to be in a media climate that mimics the McCarthy or Stalin eras.