This is one of many questions asked in this NY Times piece by Anne Barnard, which looks at possible reasons Assad may have unleashed such a deadly chemical weapon attack.

Bottom line: we may never know. But it is probable that somewhere in Assad's calculations, the idea that the world has become desensitized to the slaughter in Syria and Egypt may have played a role.

"What makes military and strategic sense to Assad may not make military and strategic sense to us," said Emile Hokayem, a military analyst at the International Institute for Strategic Studies. "Assad is fighting his own fight on his terms and on the timing of his choosing. He may have made a mistake this time -- perhaps he didn't mean to kill that many, or assumed the international community had become less sensitive -- but it doesn't mean that it didn't make sense from his perspective."

The attack, which killed hundreds of people in heavily bombarded suburbs east and southwest of Damascus, the capital, appears to have been by far the most widespread and deadliest use of chemical weapons in Syria, where toxic gases have been used in several smaller attacks over the past year, with each side accusing the other of using the internationally banned weapons.

Yet in some ways, the episode may represent more of a continuity with the conduct of this war than a departure from it. During two and a half years of conflict, Mr. Assad has slowly increased the intensity of attacks on civilian neighborhoods where rebels have found support. Mr. Hokayem calls it a strategy of "gradual escalation and desensitization" of the public in Syria and abroad.

Government forces have used blunt and imprecise conventional weapons, firing Scud missiles and unleashing artillery bombardments and airstrikes on neighborhoods, in attacks that seem aimed more at sowing fear and punishing populations than at specific tactical gains. While last week's killings appear to have been the largest mass slaughter of the war, conventional weapons have killed many times more people than chemicals.

Even after Western governments declared that Syrian government forces had used banned chemical weapons like the nerve agent sarin, crossing what President Obama had once called a "red line," the attacks provoked little visible response.

And in recent weeks, with the United States and its allies increasingly queasy about Islamic extremists among Mr. Assad's fractious opponents and the prospect that his fall would bring even greater chaos to the country and the region, Mr. Assad could watch Egypt's generals preside over the killing of more than 1,000 Islamist protesters, also with few international repercussions.

Any attack by the US and its allies will be a pinprick - "just muscular enough not to get mocked," said one US official. If that's the case, then perhaps Assad did not miscalculate, but rather sized up the situation correctly. The world may not be rooting for him to win, but they're definitely not going to deliberately create the conditions where he will lose.