Strategically, why would Syria have used chemical weapons on a ‘small scale’?

The United States now says it has evidence that Syria used chemical weapons on a "small scale" — an announcement that follows similar declarations by the French, British, Israelis, and Qataris. But the question no one has been able to answer is this: Why would Bashar al-Assad have used chemical weapons on a small scale after repeated warnings from Barack Obama that any use of chemical weapons would be a "game-changer" for the United States?

It’s a puzzle that baffled Ralf Trapp, a consultant and renowned expert on chemical weapons, who spoke with Foreign Policy over the phone from Geneva. "From a military perspective, it doesn’t make sense to use chemical weapons bit by bit," he said. "Why would the regime just put it on a grenade here or a rocket launcher there? It’s just not the way you’d expect a military force to act."

But that’s precisely the scale at which the United States says Assad is using chemical weapons. "The U.S. intelligence community assesses with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale, specifically the chemical agent sarin," said Defense Secretary Chuck Hagel, speaking to reporters in Abu Dhabi.  

From Trapp’s perspective, the tactical purpose of chemical weapons is to inflict mass casualties. Since Assad is more than capable of killing rebels in small doses, why would he deploy a small amount of chemical weapons when it runs the risk of inviting a U.S. military intervention?

It’s obvious by Assad’s attempts to court U.S. media that he desperately wants the United States to butt out of Syria’s civil war. In today’s New York Times, Anne Barnard gives a fascinating window into Syria’s charm offensive to convince reporters that U.S. support for the rebels is against its own interests. Additionally, Russia, an ally Syria can’t afford to lose, has also warned the regime against using chemical weapons.

Tactically speaking, using chemical weapons on a limited scale could instill fear in the populace. But is that benefit for the Assad regime worth the cost of provoking the United States and angering Russia?