And how it presented the Syrian army with an opportunity to attempt another encirclement of ISIS forces
As I write this the Syrian army seems to be in the last stages of re-opening the road link between its territory in Aleppo province and its main territory in the east. The link had been cut off on Monday 22nd by an ISIS offensive from the east supported by Jund Al-Aqsa attacks from the west.
The fact that in the weeks before the Syrian army had been scoring very significant victories, and seems on track to restore the link within days beg the question how come this happened in the first place.
The first part of the answer is that this is a favored and well honed tactic of ISIS. Since the atrocious loses it suffered in the defeat at Kobani, the group rarely opts to commit massive number of troops to losing defensive battles. Instead it extricate's its men from such battles leaving behind only booby traps and snipers to cover the retreat and inflict losses at a small price. No sooner than the retreat is done it then seeks to compensate for the loss of ground by striking at a different location and taking the enemy by surprise.
Indeed, just a day before the ISIS attack on the Hama-Aleppo highway it had been forced into retreat as the Syrian Army took significant new ground in eastern Aleppo province - so an attack like this should have really been anticipated.
The second part of the answer is the geography of Syria's war. Most of the country consists of empty space - sparsely populated desert and semi-desert whose only value are the few roads that interject it. In terrain like that there are no real front lines. All there is are small outposts along the road – usually in the few villages that do exist.
While these roadside bases ensure a measure of control and security they're relatively isolated and weak and can always be outflanked and overrun by a determined enemy that decides to mass and move across empty space - i.e. using a secondary road or even moving off road if the distance permits it.
The reason this does not take place far more often is that the force that wrestles control of a section of road like that is now equally exposed to counter-attack as the enemy they had taken it from had been. This is exactly what we're seeing now with ISIS on the south Aleppo road, now apparently threatened with encirclement by the Syrian army.
It was a bold move by ISIS and perhaps a necessary one to boost its morale after setbacks in eastern Aleppo but won't have strategic significance if it can't hold onto it and will have been a costly military loss if it can't extricate itself from its predicament and leaves behind trapped fighters.