There were many technical reasons for the deal’s failure, most having to do with the deception and subterfuge of the Syrian regime. But the main reason Assad did not fully implement the agreement was political. At Russia’s insistence, the deal did not allow for the use of force or the automatic imposition of sanctions even in the event of Syrian violations—depriving Obama of a credible threat of force against Assad.

It’s now clear that, in the absence of such a threat, Syria had no incentive to comply. From the moment the regime shed the first blood in what began as a peaceful uprising in 2011, there was a diabolical method in the madness of its retaliation. Assad embraced a doctrine of gradual violent escalation with deep roots in Syrian history, in a crude imitation of Caliph Mu’awiya, the brilliant founder of the Umayyad dynasty (661–750 AD) that ruled the Muslim community from Damascus for almost a century. Mu’awiya was a subtle Arab version of Machiavelli; Muslim tradition credits him with formulating the following method of dealing with potential troublemakers: “If I could use my money, I would not use my words; if I could use my words, I would not use my whip; if I could use my whip, I would not use my sword; but if I have to use my sword, I will.” But Assad, the obtuse, brutal lisping satrap of modern-day Damascus, saw only the whip and the sword in Mu’awiya’s rich quiver.

So in Assad’s Syria, initial machine-gunfire against civilian demonstrators gave way to tank shells. Then helicopter gunships began strafing rebels in rural areas, followed by fixed-wing fighter-bombers, to be complemented later by Scud ballistic missiles. Then the Syrian skies were owned almost exclusively by ugly Soviet-made helicopters spewing unguided barrel bombs, laden with shrapnel, to kill and maim mostly civilians. Finally, Assad unsheathed his ultimate terror sword: chemical weapons.

Still, after each escalation, Assad would pause briefly and gaze westward, waiting for the reaction of the Obama administration and the European powers. He took Obama’s measure early on, and realized that the American president, who was cognizant of the limits of his country’s power and who was very eager to withdraw from Iraq, would limit his reaction to eloquent statements of moral outrage and righteous indignation.

Obama issued his “red line” warning to Assad in the summer of 2012, not long after regime acknowledged for the first time that it possessed chemical weapons. That winter came the first reports alleging the Assad regime’s use of those weapons in Homs. In that incident, seven people were killed, and eyewitnesses described victims’ symptoms of breathing difficulties, nausea, and blurred vision. On January 15, 2013, Foreign Policy reported on a secret cable from the U.S. consul-general in Istanbul, Turkey, who claimed there was compelling evidence that the Syrian military had indeed used chemical weapons there. The following day a spokesman for the National Security Council played down the report. But by spring 2013, there were many reports of chemical weapons being used by the Syrian military, prompting Obama, members of congress, European diplomats, and then-UN Secretary-General Ban Ki-moon to call for investigations.