The military intervention by Russia on the behalf of the Assad regime has led to a rapid reversal in the course of the fighting. Pro-Regime forces have made sizeable gains on the ground and are on the verge of circling Aleppo, Syria's biggest city. The western media has swung from predicting Assad's imminent downfall to predicting an eventual victory for his forces. This swift turnaround in predictions ignores one central point in all foreign military expeditions in the modern era which is summed up in David Petraeus's comment on American policy on that country's occupation of Iraq; "tell me how this ends."
It is entirely feasible and indeed likely that with a combination of indiscriminate Russian air power and foreign Shia troops that pro-regime forces could defeat all the rebels including ISIS and occupy much of the territory lost in the civil war, but maintaining order would entail permanent military occupation as the conflict would undoubtedly shift into one of guerrilla warfare where control of territory is less important than grinding down the occupying army in terms of men, morale and money following Mao Tse Tung's maxim that guerrilla warfare is about "trading space for time". As conflicts from Vietnam and Afghanistan in the Cold War era to our generation's post 9/11 war have shown, winning every engagement is no good if the population is resolutely against you. Sooner or later your bank account or patience runs dry.
In the context of Syria, there is the added frisson that Assad's indigenous armed forces have been degraded to such an extent that it would be impossible to maintain control over the country without most of the boots on the ground coming from Assad's foreign allies. What would result would be the sectarian repression of Sunni Syrians in their own country by foreign Shia and Christian troops broadcast throughout the Sunni world by Al Jazeera and spreading virally on the internet. It is not an exaggeration that such a scenario would lead Syria being a source of anger among Sunnis equal to the Palestinian situation, perhaps more given the novelty of the event is likely to increase the injustice that young Muslims feel over it.
Short sighted commentators in the West commend Putin's actions in backing Assad in Syria as they believe it will keep us safe from ISIS. If anything it will increase the threat that ISIS poses to us by reinforcing their narrative of a cosmic war between believers and infidels. In their earlier incarnations in the late 2000s, ISIS have had much experience of existing with little to no actual territory controlled, and not only would still be able to organise terrorist outrages in the West but would find that their level of support may rise as they would be able to pose as the only defender of Syrian Muslims, especially if the Russians efforts to destroy the moderate opposition are successful.
The streets of Britain will be less rather than more safe if Assad achieves a 'victory' over the rebels. Isis will have a powerful new recruiting tool and tensions between the Sunni and Shia communities in the UK, may reach the level of tension of that between Muslim and Jewish communities. If Syria does turn from a 'war of space' into a 'war of time' then the after effects of the conflict could affect us here in Europe for several generations.