Twelve years ago, Syrian President Bashar al-Assad’s government embarked on a top-secret mission to produce large batches of mustard gas, a crude World War I-era blister agent that Syria manufactured as part of a broader chemical weapons deterrent against militarily superior enemies, including Israel.

Between 2004 and 2007, Syria made some 385 metric tons of sulfur mustard, enough to fill thousands of artillery shells. But Syria has admitted to building only 15 Scud missiles capable of delivering 5 to 6 metric tons of the chemical agent, leaving a yawning gap that has left weapons inspectors questioning whether Syria may have retained a stockpile of tactical chemical munitions it has never acknowledged. That’s the conclusion of a highly confidential, 75-page report by the Organisation for the Prohibition of Chemical Weapons (OPCW) reviewed exclusively by Foreign Policy. Continue reading.