Jon Western is right to point out that there are certainly cases of intervention success. Indeed, those arguing in favor of intervention in Syria will surely draw on cases like Bosnia and Kosovo to make their case for intervention in Syria.
Setting aside the possibility that cases like Bosnia and Kosovo are not great examples of intervention success, there is certainly some persuasive evidence that military interventions can work. The cumulative research from people like Page Fortna, Barb Walter, and Michael Doyle & Nicholas Sambanis suggests there are some important qualifications to this.
First, the interventions need to take the form of peacekeeping missions—with well-resourced boots on the ground to protect civilians and enforce the peace.
Second, the missions must be multilateral, providing further credibility to enforcement and legitimacy to the cause.
Third, the peacekeeping missions must be multidimensional. This means they aren’t just military missions, but that they also involve extensive efforts at state capacity-building, humanitarian assistance, refugee resettlement, economic development, election monitoring, and the like.
Fourth, the combatants are ready to negotiate and consent to the intervention. Although there is some controversy about this (Fortna finds that Chapter VII enforcement missions are just as effective as consent-based ones), Doyle & Sambanis suggest that multilateral peacekeeping missions aren’t too successful at ending civil wars; rather, they help to enforce the peace once hostilities have ceased.
All of these conditions being present, there have been some peacekeeping successes (e.g. East Timor, El Salvador, etc.). Any of these being absent, the results are more mixed. All of them being absent, the outcomes of international intervention are much less favorable in both strategic and humanitarian terms. See several of my previous posts (here, here, and here) for more on this.