There's wild speculation that after the Boston bombing Obama and Russia's Vladimir Putin made a deal; Washington lets Moscow do whatever it wants in Chechnya like, forever, but gets a nod to install a "no-fly zone" and further mayhem in Syria. There's no evidence for that. What a geo-politically savvy Putin wants to know is what does he get out of Syria in practical terms (and Obama does not have a clue). Crumbs from a NATO banquet don't apply.
As for allowing Syria to become a "Western-friendly" Wahhabi emirate or yet another failed Muslim Brotherhood fiefdom, one needs to go no further than Hezbollah's Sheikh Nasrallah ... "the goal of anyone standing behind the war in Syria, is destroying Syria so that a strong, centralized state would not be established in it, and so that it would become too weak to take decisions related to its oil, sea, or borders."
Now that's what a red line is all about.
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