Much as it struggled to understand the weapons capabilities of Saddam Hussein's Iraq over the years, the United States is now bedeviled by a growing body of evidence that suggests Syrians have been exposed to chemical weapons at least twice.
Far from clear, however, is whether the evidence conclusively shows that Syrian President Bashar Assad has crossed what the Obama administration has called a "red line."
The United States and its allies have been pressing the Syrian government in recent days to allow access to a U.N. team of inspectors, but Damascus has not agreed.