Perhaps out of logic, straightforwardness and/or a desire to meet the other halfway, President Obama appears to have a tendency to make commitments that later come back to bite him.
In the first instance, on Sept. 25, 2012, the president stated publicly that "containment is not an option" in the event that Iran did not renounce its development of chemical weapons. In other words, and in the event of a worst-case scenario in which negotiations completely fail, Barack Obama has committed himself to an unprovoked military attack on Iran, which would have a disastrous effect on world public opinion and lead to unpredictable human and material damage.
It is difficult to see, even if he so desired, how Mr. Obama can easily back away from this commitment. Possibly there could be a break in the situation, and a move toward peace, in the aftermath of the Iranian presidential election in June, but this is only a guess at this point.
In a second instance, on March 20, 2013, the president publicly reaffirmed an earlier statement that if Bashar al-Asad made use of chemical weapons against the Syrian rebels, it would be a "game changer" that would cause the U.S. to take unspecified a action against the Syrian regime. Here the president has some wiggle room. Which is to say that providing arms to the rebels would be a game-changer. The U.S. has said that it has not provided lethal weapons to the rebels. Whether or not this is completely true, it has certainly not taken place on a large scale.
Providing arms to the rebels would not be an unduly provocative act vis-à-vis the Russians, as they are doing the same thing for the Bashar regime. Presumably there would be no "no-fly zone" and no American boots on the ground. There are signs in the recent days that the administration as begun to consider seriously this option, as reports of the Damascus regime's having used chemical weapons at least once are beginning to filter in.