Skeptics about humanitarian intervention in Syria hit you with what they regard as a killer question: "Where do you stop?"
If the "international community," such as it is, tries to halt the massacres in Syria, why doesn't it intervene in North Korea or Somalia? If the political partialities of your inquisitor lean to the pseudo-left, the whataboutery does not stop there. Guantanamo, drones, the West Bank or whatever else is troubling them that day mean that nothing can be done for the Syrians until the lands of the West have been cleansed of their sins.
The only proper response to "where do you stop?" is "when do you start?"