According to John Kerry, the case for bombing the Syrian regime is to keep Assad from using chemical weapons again. Similarly, I hear from a lot of liberals is that the sheer moral offense of using chemical weapons obligates the United States to attack the Syrian regime.
But 100,000 people have already been killed just as dead with conventional weapons, which can be just as indiscriminate as the worst poison gas. (Consider the estimated 100,000 civilians killed in a single night’s firebombing of Tokyo.) So put another way this amounts to sending a message that, should you be a dictator slaughtering your own countrymen, make sure to slaughter them with conventional guns, bombs, and knives instead of chemical agents.
Ezra Klein puts it well:
This is why there is no enthusiasm for intervening in Syria: Making the decision to punish Assad for using chemical weapons means explicitly making the decision not to stop him when he slaughters with conventional weapons. The brutality of what we are willing to accept tarnishes the better world we seek to preserve.
Now, in addition to that, there’s the fact that according to the UN charter (which the US has signed) it’s illegal to attack another country without a UN resolution. There is a treaty banning chemical weapons too, but Syria has not signed it. So we’d be violating a much more important international law to protect a tacit norm of moderate utility at best. The way to preserve international norms and institutions, in my view, is first to not violate them all the time oneself.
Furthermore, it’s quite unlikely that other countries will look at Syria and conclude that one can merrily gas away and get away with it, much less gas the United States, the world’s most aggressive and powerful country, as the administration has ludicrously suggested. It’s obvious that everyone is reluctant to get involved in Syria because of the idiosyncratic nature of the civil war there—it’s got everything short of “QUAGMIRE” written in the sky above it in 50-foot letters.
So let’s stay out of this.