"We can't be spectators to slaughter, except for all those other slaughters"
The essence of this argument is that, because we did nothing about Rwanda, nothing about the slaughter in the Congo and next to nothing about the endless series of ethnic wars perpetrated in Sudan, we don't have the right to do anything about Syria.
Put this baldly, the argument seems (I hope) idiotic. But like the claim that we can't trust Obama because we couldn't trust Bush and Cheney, it is a principal refuge of liberals who don't want to see the United States do anything in Syria.
Well, I guess if you buy the notion that, once you have done the wrong thing, you are never allowed to do the right thing, this might make sense. But I don't think that many of us believe that people can never change. In fact, when I hear either one of these arguments, what it makes me suspect is that people who resort to this line of reasoning do not have any substantial reasons to sit and do nothing while innocent people are slaughtered.
This doesn't mean, by the way, that I support military action against Syria. I still haven't heard a viable suggestion about what resources of the Assad regime could be destroyed without tipping the balance of power in Syria to the current opposition, which seems to have been (as far as I can tell, seen through the lens of our miserable press) totally co-opted by the worst sort of revanchism and religious extremism. But if we oppose Obama's intentions, let it be on the basis of real issues, not patently irrelevant knee-jerk reactions.