Well, I started this, so I'll follow through. Whenever I come up with an idea that sounds really smart to me, I tend to come up with potential arguments against it. So here's a few I've considered:
Q) Why even bother? ICC prosecutions take forever.
A) I know they do. That sucks. I wish justice was faster. It's not even fast within the same country, but it's especially slow internationally. But the question is: compared to what? Invading Syria? Launching a few random cruise missiles or drone strikes? What alternative will have an impact on what's happening right now?
Q) What if it wasn't Al-Assad's idea to launch chemical weapons? What if was some random lieutenant, or even the rebels?
A) That's why we have a trial. Instead of rushing off and launching missiles based on what we think happened, we can find out who's actually responsible, and prosecute them.
Q) But lots of American leaders have done things at least as bad.
A) I know. And if this was a just world, George Bush, Dick Cheney, Donald Rumsfeld, Henry Kissinger and a lot of other American leaders (maybe even You Know Who) would be sitting in cells in the Hague right now. But it's not a just world. It's the one we live in. The choice isn't between prosecuting Al-Assad and prosecuting George Bush. It's between prosecuting Al-Assad and killing some random Syrians to teach Al-Assad a lesson. So which do you like better?
Q) How do you even prosecute a person in the ICC?
A) I have no idea. I know there are people that do. I know we are the most powerful nation in the world, and if we want to make it happen we very likely can. But it won't if we don't try.
Q) Are you sure it will work?
A) No. And if it does, it won't work for a long, long time. I'm saying that there is no idea out there that is better. The reason I'm proposing this is because if you say bombing Syria is a bad idea, people who want to do it will say, "Well, what would you do?" So now you can answer the question.