to air and missile strikes, possibly with training and resources being provided to rebels in Syria. That's certainly not a good thing, but it's not cataclysmic. There's also a strong possibility that this is a calculated bluff, to shake up the status quo. NATO has the ability to destabilize the military strength of the Assad regime, by taking out their air capabilities and key military infrastructure. They could enforce no-fly zones and target Syrian forces that are attacking. Libya is probably a good model for what could happen, but likely military action would be less severe, since there isn't much clarity on how to effectively replace the Assad regime.
It's definitely very strange that Assad is apparently continuing to use Sarin in attacks against civilians. Either he's crazy, he doesn't have control over the Syrian forces that are using sarin, or he's desperate. He may be overconfident of Russia's support.
By the same token, military action against Syria is pretty crazy, since things are so unstable. There aren't many foreseeable good outcomes to getting rid of Assad, and things are already unstable enough to be having effects beyond Syria's borders. Israel is clearly concerned, and watching the situation closely.
The level on which this makes the most sense to me is as a very credible bluff by NATO, to try to shake the situation out of its current unstable rut. This could impel Russia and/or neighbouring countries like Iran to take a more active role in negotiating a better solution for Syria. It could promote a shakeup within the Assad regime, with leadership passing to more stability-oriented individuals.
Even if the bluff is called, NATO military action will be limited. It's very hard for me to imagine anyone putting boots on the ground in Syria - that would be a logistical, financial, political, and diplomatic nightmare. This will be an air war - minimal casualties, taking out Syria's air defenses, which will leave them much more vulnerable to potential future military actions. Russia will grumble, but there's not much they can do. Military action in Syria isn't going to significantly shift public opinion polls anywhere.
I'm not saying this because I like it, or I support it. I can't really say whether I think it will be effective or not - I think it's potentially effective as a bluff, but don't have adequate information to evaluate the effectiveness of a military action - I haven't been following this closely enough.
I feel that this is one of the repercussions of the Arab Spring - the Middle East is in a less stable phase, but the previous stability was based on supporting repressive dictators. The instability that we're seeing now in the Middle East is at least partly a reaction to that repression. There really aren't graceful ways to transition.
I've been reading various theories on DK about what military action in Syria would mean, so I decided to share my perspective. I'm not defending/supporting military action, and I don't have any alternative solutions. I don't think that the worst-case scenario in Syria comes anywhere close to the consequences of the US wars in Iraq and Afghanistan.
FWIW, here's a Slate article that I found interesting on the topic: Obama’s Guns of August. It doesn't much discuss the geopolitical factors, which I'm interested in learning more about.
Update: Several commentators have made the case that Syria's air defenses are quite formidable, and could provide a much stronger defense than Libya, for example. That could be a significant factor.