The nature of the opposition movements makes this an unwinnable war.  The lack of coherent and unified opposition in Syria means that a Western invasion force will have very low chances of meaningful success.  Western intervention always carries the risk of fracturing a conflict by splintering opposition movements into those who do and those who do not support Western involvement in achieving their cause. This is problematic at best. With Syria specifically, this issue is augmented further still due to already existing lack of coherence and unity in the opposition movements. Absent a proper hierarchy and structure the opposition movement is going to be near impossible to cooperate with and will quickly splinter into an insurgent-style conflict.  There are multiple issues with this. First, there is very low chance of success in such a situation. Second, this set-up is the type that is most likely to lead to a long, protracted conflict that does not serve the interests of the West or the people of Syria.