For a smart quantitative take on the effects of a Syria intervention, head over to the cryptically-named bennstancil.com (presumably operated by one Ben N. Stancil).
[D]o existing civil wars become more violent after an intervention?
The limited historical record indicates that it’s relatively rare - but it’s even less rare that they reduce violence. A conflict with an intervention was preceded by a civil war in 28 instances. In four cases, the conflict became more violent during the year of the intervention compared to the year prior. Twenty-three stayed the same, while only 1 declined. (Including conflicts that are already major conflicts in this sample is potentially problematic, however, because these conflicts have already reached this dataset’s upper bound of violence. For minor conflicts, 4 of 16 increased in violence, while zero decreased.)As the post accurately points out, this doesn't do much to explain causation. It may be that intervening in civil wars makes them more violent, or it may be that foreign military powers are reluctant to get involved in any but the worst civil wars. Either way, though, it's just more hard evidence in support of what everyone already fears: the Syria situation is a disaster and it's unlikely to get much better anytime soon.
Violence, however, isn’t the only measure of a conflict. There are other potential benefits to interventions, such as bringing the conflict to an end more quickly. From this perspective, history isn’t in favor of the interventionists. Only 8 of the 28 conflicts ended in two years following the intervention, with many lasting much longer (75% of those that endured lasted at least seven years following the intervention year). Furthermore, conflicts don’t appear to end more quickly if the intervener remains present either - in the 20 cases in which conflict endured, 10 reverted back to civil wars, and 10 continued as interventions, with roughly equal portions of each lasting seven or more years.