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.