The answer clearly depends on internal and external factors. The most important internal factor is how the Syrian establishment handles the victory. Has it learnt from the errors committed 3 years ago, when the protests started, and will it go for reconciliation, inclusion and democratic opening, or will it simply use the victory to reestablish the old elite's hold on the power. If they opt for the latter, this will be a very long drawn-out conflict that will burst into open rebellion again at some point. There are at least some indications that changes will come about, perhaps as the military takes on a more powerful role, reducing the influence of the brutal and dreaded secret police. But truth is that very little is known about it. Those who want to reestablish the old order, will probably take inspiration and comfort from the recent military coup in Egypt, paradoxically supported politically both by the US and Russia.
However, even if some degree of reconciliation is achieved in Syria, what will the reaction of the outside powers be? The Syrian Government has very powerful (and rich) enemies: Qatar, Saudi Arabia, Turkey and – not least – Israel and the US. If these countries choose to react in the same way as in other circumstances, e.g. Afghanistan after the Soviet military intervention, or Nicaragua after the Sandinista revolution in the 1980ties, this can be a very long drawn-out affair with a costly low-intensity war.
During the 1980ties the US armed a military rebellion in Nicaragua called “the contras”, and provided them with bases in neighbouring Honduras. The Honduran government never officially acknowledged the existence of these bases, despite a lot of press reports and even visits by US politicians to encourage the freedom fighters. The bases served as a sanctuary for the rebels, untouchable for the Nicaraguan military, as that would be considered an attack on a peaceful US ally. So rebel troops would venture over the border and then withdraw back to Honduras, when counter-attacked by the military. The idea was originally to establish a bridgehead on Nicaraguan territory and then install a competing government there, which could be recognized by the “international community”. Even if that aim was never achieved, as the Sandinistas, with support from the extinct USSR, were too strong politically and militarily, the damage caused to the country was enormous with a relatively small investment compared to an open US military intervention.
There are signs that a similar strategy is already being implemented in the case of Syria. This is done from two territories: Turkey and Jordan, both close US allies, and in the case of Turkey, protected by its NATO membership. Turkey has long served as a corridor for foreign Islamic fighters, and supply line for the rebels. The stakes have been increased recently, as Turkey has let the rebels make an attack over the border to attack the Armenian village of Kessab, close to Latakia, and according to at least some information with logistical and military support from the Turkish army. The attack, which has no chance of seriously threatening the Syrian army's grip on the province, looks like a – partially successful – attempt to divert army forces from other combat zones. Something similar has long been expected on the border to Jordan – a much announced rebel “spring offensive” – but has yet to materialise, despite the establishment of a joint rebel-NATO command centre in Jordan. So it looks very much as a re-run of the Nicaraguan “contra” war.
But does this mean this that US and its allies – including NATO – have already taken the decision to go for the long low-intensity warfare to wear out the Syrian Government? It is difficult to know, and it may depend on whether direct talks can be established between US and Syria, and whether the US conditions for such talks will be palatable for the Syrian Government. If the talks start with a US condition that “Assad has to step down”, as they did in Geneva, these talks obviously have no future. But if focus instead could be moved to some sort of internationally monitored elections, return of the refugees and rebuilding of the country, perhaps there is a way forward. Alas, chances for such an outcome are not big.