Putin's actions must be based on the assumption that Obama will not act, right? Or that, if he acts, out of a sense of looking beaten by Putin and fear of how that might roil power dynamics in the region, his actions will be, as Kerry emphatically said, 'incredibly small' and have no effect, thereby making Obama look reckless, weak and uncomfortable with or even contemptuous of the traditional strategic uses of American power, an impression that would also roil power dynamics in the region. Because if Obama rejects Putin's no doubt hollow offer to broker a deal with Assad on surrendering his WMDs and launches significant strikes that actually threaten the viability of Assad's regime, well then Putin will look the loser, yes? Or would he have some other trick up his sleeve should that happen?
I'd say Putin must feel confident about three things here: that the UN will point blame at Assad for the chemical attack but that there'll be no beyond reasonable doubt proof [remember, Russia must have inside info on all of this]; that the EU will be very worried about escalating this mess and will jump on his offer no matter how dubious it is, thus isolating America; that, caught like this, Obama will have no stomach to meet the challenge Putin has tossed in front of him and instead will look for a means to talk his way out of his predicament.
What about trying to trap Putin at the UN with a counter offer that says if Assad doesn't surrender his WMDs within a certain time frame punitive military action will be authorized? I assume at that point Israel's possession of WMDs would enter the fray and scuttle everything. But wouldn't that show Putin's plan up as having been a ploy? Possibly - but again he may be simply looking for two things here - sucker in the EU to accept anything in order to wriggle out of this mess, and stretching out the timeline which increasingly makes a punitive attack look ridiculous. And remember as well, verifying stockpiles and consequent removal of WMDs in a country not at war is difficult, so one has got to figure everyone starts off here knowing that Putin's offer is bullshit and so it really becomes a question of how desperate are the various parties to crawl out from under the mess they've made. Putin I guess feels they're very desperate to do that.