The region is pretty unstable as it is, but obviously any form of US intervention in the Syrian civil war carries great risks. In this Putin is at least more right than the Obama-administration whose assurances that this would be a strictly limited operation without excessive risks is either disingenuous or just plain stupid.
At this point I see no conclusive evidence for who used chemical weapons where. Putin's assertion that it was the rebels is as unproven as Obama's assertion that it was Assad.
On the whole, it is preferable to use diplomacy rather than military force, if essential goals can be reached this way. But diplomacy and military force are both tools. It's the policy that matters.
One can conduct a whole semantic discussion about American "exceptionalism". I'm not a big fan of the term. But the fact is that the US is an exceptional global superpower and has been - on the whole - an exceptional force for progress and good in the World.