I'm interested in how folks here would consider the situation in Europe under the Nazi boot. Should other nations stand back and tut quietly whilst millions were being gassed, shot, starved, experimented upon, etc? I'm not talking about the fact that we were already at war, I'm considering the moral arguments.
I'm against intervention in Syria because there are no clear "good" guys. Both sides have shown murderous intent and a rebel victory would spell the end for all sorts of minorities who could survive only in the relative stability of Assad's police state, so it's certainly not a clear-cut "intervene to save the xxx people from genocide". When there is no obvious right answer, the case for respecting the will of the international community is much stronger.
In Courtney's example there is one protagonist setting about a genocide, but with no threat of counter-genocide from the other side. There is thus a much clearer moral imperative to intervene on the side of the victims, and while achieving international consensus is never likely to be 'easy', it would surely have been more straightforward than in the Syria example. We saw this in the Balkans, where the Serbs showed un-reciprocated genocidal intent.
So, I think it's perfectly possible to have a consistent moral viewpoint that leads one to be pro-intervention in some cases, and anti-intervention in others. There's no black and white in this argument!
Edited to add: The "good" side will never be whiter-than-white; the trick is to see through the actions of rogue elements and identify the true intent behind each side's actions.