Inching towards the end

"Until early this year two of Syria’s players, the regime of Bashar Assad and Islamic State (IS), were in the ascendant. The tide against both has since turned. Recent analysis by Jane’s, a defence analyst, suggests that after a series of losses to both IS and other, rival rebel groups, the area fully controlled by government forces has shrunk since January by some 16%, to a mere 30,000 square kilometres (11,600 square miles), or barely a sixth of Syria’s territory.
 And this was before a recent offensive by a coalition of mostly Islamist militias calling itself Jaish al-Fatah, which on July 28th captured the government’s last salient on the route linking Syria’s second city, Aleppo, to the coast. The advance, which follows the fall in March of a provincial capital, Idlib, and of another big town, Jisr al-Shughour, in April, further consolidates rebel control of Idlib province. Jaish al-Fatah now threatens both the surrounding rich agricultural region and the coastal mountain range that is the heartland of Mr Assad’s own Alawite sect.
 Despite the fact that Turkey has long argued for a stronger international effort against Mr Assad, its formal entry into the fray is unlikely to tip the equation decisively against Syria’s regime. Iran, soon to be flush with funds unblocked by the lifting of sanctions, is for now at least still committed to propping up the Syrian leader. Even with better weapons and stronger momentum, Sunni rebels lack both the manpower and unity of command needed for a push towards Damascus, the Syrian capital. Some of their component groups, such as Jabhat al-Nusra, which declares allegiance to al-Qaeda, remain anathema to Western powers who might otherwise be keener to accelerate Mr Assad’s fall.
 Despite its success at Palmyra, IS’s “caliphate” is an unhappy place, bludgeoned from the air by coalition strikes and shrinking in size. Kurdish forces have relentlessly expanded since breaking the jihadists’ siege of Kobane last winter, most recently seizing the town of Sarrin on the Euphrates River. While few expect any sudden collapse, many analysts believe that, barring some error by its enemies that would win it a flood of new allies, IS may have already peaked in size and strength."
 It might be said they can't push towards Damascus because the Americans won't allow them to.