U.S. intelligence has detected increased activity at Syrian chemical warfare facilities, raising concerns about the regime potentially using chemical weapons (CW) against the opposition. Although such an action would likely only be carried out in extremis, Bashar al-Assad and his cohorts are approaching that very status. Given the regime's lack of regard for the casualties it has already inflicted and the value it places on its own survival above all else, the United States must prepare for the growing possibility of CW use in Syria.
WHY NOW?
After twenty months of internal war, the regime is in an increasingly difficult military situation. It suffered substantial reverses in November, losing territory, positions, troops, and equipment, including combat aircraft. The fighting capacity of its forces appears to be diminishing, and its standard tactic of bombing and shelling opposition areas, primarily civilian, is becoming less effective and more costly due to rebel antiaircraft fire and other tactics. As a result, pressure is undoubtedly rising within the regime to take different and more effective action -- that has been the pattern for Damascus since the emergence of armed opposition in summer 2011.
http://www.washingtoninstitute.org/policy-analysis/view/how-would-assad-use-chemical-weapons