[A philosophical follow-up to the previous post]
Imagine, for a moment, that you could only call the police twice in any one year. How would this affect your decision making? Even if you were not prone to calling the police over every little thing, it would, hopefully, make you consider what is a real emergency that would require police intervention. But it would have to be carefully thought in time considerations too. If, after all, you witness two police level emergencies, one in January and one in February, you'd have to think about whether the second one is one you really want to call on, or whether you should wait it out and see if there's an even bigger one that comes along. After all, use up the second one in February and you'd have to go through the whole rest of the year not being able to call the police.
Now this doesn't preclude other possibilities; you could ask others to call on your behalf. But it would require convincing others to use up their allotted uses of the police. And you would have to find a person and convince them, all the while the crime is still on going. Ideally there would be a way of coordinating everyone, but given the usual game theory dynamics, such coalitions could end up being unstable (i.e. everyone would want to keep one back to reserve for harm done to them, and let others report the crimes they see done to everyone else, so ultimately nobody does anything).
Naturally this means that you'll have to pass up on reporting some pretty horrific crimes, or hoping someone else reports them, because you have to sit in anticipation that you might witness an even worse crime. It'd be difficult, certainly. Immoral? Perhaps as well. But that's the constraint of the system.
The punchline to this thought experiment is, of course, the perilous world of humanitarian interventions. The thought experiment is reductionist, sure, and a little unfair, in that (like all thought experiments) the deck is stacked to lead towards a specific answer. But the purpose of it is to illustrate a point: that you have a finite amount of resources. Both in terms of finance and goodwill. You need both, the money to do so and the public goodwill, to be able to engage in any kind of humanitarian intervention, particularly ones that will take a long time. So you need to be careful in picking them. Otherwise you might use up all that goodwill and resources on one that, on reflection, would have merited not being involved in.
The punchline to this: Iraq and Syria.
Consider: why is there no appetite for more action
on Syria? Why it's thanks to Afghanistan and Iraq. You see resources are
finite, goodwill is finite. Blair and Bush used up everyone's good will
and hence why there's so much reluctance to get involved in any real
way in Syria.
Of the cases I would say that Syria merits a full-scale humanitarian intervention much more than than the other two. However Syria can't get one, because the resources were already used up on Afghanistan and Iraq. The point being that for anyone who supports interventions (and I'm a shade on the support side - but a whole raft of conditions) they need to think carefully about this problem. That sometimes we might have to sit on our hands. Pick your battles carefully so that you're only doing it in the cases that really, really merit it. Because otherwise you might waste one and then not be able to do another that merits it more.
This is a point that a lot of interventionists don't seem to grasp . Frequently when you hear denunciations about how 'inaction is also and action' it misses the important point that; A) it's a not a choice between 'inaction' and 'any action' or 'the perfect action', but a choice between 'inaction' and 'this specific action'. In some cases inaction may be the preferable choice.
And B) it misses the point that actions are not made in isolation. After one war stops, the world doesn't reset. Action is a form of action and actions have consequences. The consequences of that action persist; and it effects the next choice and limits the actions that can be taken. Full scale intervention is not on the table in Syria because the resources for that were used up on Iraq. Hence we have the watered down intervention now that isn't really doing any good.
The serious point to this is that anyone advocating intervention needs to think hard about what justifies a case. And yes there will be many a horrific scene that is witnessed and perhaps passed over. But there might be a more horrific one around the corner. And you need the resources and goodwill for that one. Because, regrettably, the real world is not like The Lord of the Rings. If you defeat the Dark Lord order doesn't just spring back to the land. It takes time, and a lot of effort and commitment on the part of the interventionists. With intervention comes responsibility.
And consider: we've been in Iraq for over ten years now, and it's still nowhere close to being stable. Think how long Syria might take. Think how long situation X might take.
This stuff isn't easy. And nobody gains anything from people pretending that it is.
 Indeed, I'd go so far as to wager that there's a not insignificant overlap between 'people who sneer about magic money trees and bellow about the importance of being tough on debt and deficits' and 'people who insist we should get involved in every conflict going'.