The New Republic has a detailed article on how the Obama Administration has been outmaneuvered by the Russians and Syrians as to strikes against Syrian chemical weapon sites. But I think what is key is why it was possible for the Russians and Syria to do so:
Fifth, and most important, is the fact that Assad giving up his chemical weapons was only part of the stated objective. If you listened to the White House pitch closely, the point of the military strike was not just to stop Assad from using chemical weapons further on his citizens, and it was not just to warn other rogue leaders with their fingers on various triggers. Part of the goal was to force a political solution that would remove Assad from power. That is, even though the Obama administration has been insisting that it is not interested in "regime change," that disastrous cornerstone of the Bush era, it was, in fact, pursuing regime change, at least until Monday.
On August 21, just hours after the sarin attack in Ghouta, a Damascus suburb, had occurred, Deputy Press Secretary Josh Earnest spoke of the failure of international pressure to achieve a key administration goal: "We've seen evidence and indications that the Assad regime is feeling that pressure, but you're right that we have not—that it has not resulted in the outcome that we would like to see, which is Assad being completely removed from power," Earnest said. "That’s not just the preference of the United States of America, that’s the will of the Syrian people and that’s why it's important." This was what Senator John McCain managed to pry from General Martin Dempsey during the Senate Foreign Relations Committee hearing last week, that the goal of a military strike was "to change the military equation on the battlefield," and what he worked into the committee's resolution to authorize the use of force in Syria. ...In other words, it was the Administration's basic dishonesty with the American people--that this strike was because of the chemical weapons, rather than a matter of "regime change"--that made it possible for Russia to jump on Kerry's offer of Syria giving up its chemical weapons.