Do you know why poison gas isn't used by major nations?

[alcibiades_mystery]

5. The evidence for tactical advantage in front-oriented warfare is certainly limited

Keegan is right on that. But there is also an odiousness involved that is undeniable, and undeniably linked to general deterrence. In wars of attrition or asymmetrical warfare, tactical advantage can be sidelined or can be hard to discern: it's not "front" oriented, but psychological. Even on the Western Front, the mistake of people like Keegan is to think that every move is oriented towards landmass and the "Front." Not always. Plenty of morale-oriented strategy involved there, too. So your reading - and Keegan's - is overly simplistic. That chemical weapons can be of limited effectiveness in a front-oriented attack doesn't mean that they can't be effective in other ways, like reducing the will of the enemy to continue, or depleting their base of civilian support. If the latter is the reasoning of the Syrian military, your point becomes immediately moot: perhaps the chemical weapons were deployed merely to 'clear" out the civilian support in that suburb, leaving the insurgents with no "people" among which to be the "fish in water," as Mao says. Keegan's problem is that he makes a fetish of the frontal assault, always. It's his weakness in analysis.

So, perhaps what deters people from using chemical weapons is not their limited tactical effectiveness for frontal style military maneuvers, but rather something else - a general deterrence, another sort of fear.

Juan Cole has reviewed the cables, and inferred that the Syrian military may have deployed large amounts of chemical agents by accident - largely because they have tended to deploy smaller amounts for various practical purposes. That is, where "lacing" CS gas with some sarin has been effective against smaller rebel contingents, the action of August 21 saw larger amounts of sarin deployed by mistake or at a lower level of command. That's plausible. Lots of stuff is plausible. But falling back on the supposed tactical ineffectiveness of various chemical deployments on the Western Front is not really a good answer. There are many reasons to deploy gas that have nothing to do with "taking the next trench."

If that was all there was to it, you wouldn't need international laws against it - it would die quickly of its own accord. For this reason, I find your point simple and, frankly, somewhat silly.