(Or any other nation)
The primary motivation for American military intervention in Syria seems to be to save civilian lives.* The opposition is often portrayed as having other motivations, such as the intervention not being sufficiently in American interests. However, I think there is a strong case that even if you primarily want to save Syrian civilians, you should actually oppose American military intervention, too.
It is easy to imagine a simple world where people are dying and America intervenes to save their lives. That world does not exist. The real world is more complicated, and there are a number of questions an interventionist should consider before committing strongly to such a position.
1. If America intervenes militarily, how many civilians will be killed by the existing parties? Could intervention to help one side make the other more desperate, reckless, or brutal? Could intervention give victory to a side that ends up slaughtering civilian dissidents the way it slaughters military captives? If Assad goes down, will his chemical weapons get strewn about the country into the hands of who knows who?
2. If America intervenes militarily, how many civilians will be killed by outside parties? Will Iran attack Baghdad? Will Russia send weapons to Assad that eventually kill the civilians we saved from chemical weapons?
3. If America intervenes militarily, how many civilians will be killed by Americans? This is the most uncomfortable set of questions, but the most important set to consider. If we strike with missiles, by ship or by drone, will any of them miss their carefully determined targets and strike civilians? If we send troops, will any of them kill children or rape women while they’re dodging rebel fire?
It is theoretically possible, though in my opinion extremely unlikely, that the answer to all of these questions is No, none, zero, never. It is extremely likely a fact that American military intervention will kill civilians. It is possible that this intervention will save civilian lives on net – though I think even that is highly uncertain – but even in the best case we are essentially talking about killing some civilians to save many more.
This subtly utilitarian argument could be defended, but the average interventionist seems to be imagining a black-and-white world of “civilians are dying, we need to stop it,” not a more realistic “civilians are dying and here’s why I think our intervention might save more of them than we might kill.” That position is harder to argue, perhaps because most of us aren’t actually that utilitarian, at least about murder; maybe the ends don’t ever justify the means. But the point is that even if you are utilitarian here, you need to answer all three of the above types of questions to convince me that your intervention will save more lives than it destroys. Human life is too valuable to settle for less.
*Technically there seems to be an argument that Assad must be punished for using chemical weapons, although there seems to be some reasonable dispute that we know for certain that he was the one that used them, and at any rate the whole argument for punishing chemical weapons use seems to be that they can be used to kill large numbers of civilians, which puts us back where we started.