It's sad that the top comment here is a conspiratorial hypothesis with game theory as a supporting argument. It seems wrong to cite game theory as evidence that a gas attack was unnecessary when that gas attack achieved victory by forcing the Douma rebels to, just hours later, agree to surrender. It's sickening to contemplate, but one could argue that in this case, the despicable use of chemical weapons brought a remarkably swift end to the fighting in Douma and in the longer term actually saved lives in that region of Syria.
http://www.dw.com/en/syria-rebels-evacuate-douma-under-new-a...