Gas was not used in World War II. The agreement, though not without antecedents, was largely a tacit one. It is interesting to speculate on whether any alternative agreement concerning poison gas could have been arrived at without formal communication (or even, for that matter, with communication). “Some gas” raises complicated questions of how much, where, under what circumstances: “no gas” is simple and unambiguous. Gas only on military personnel; gas used only by defending forces; gas only when carried by vehicle or projectile; no gas without warning—a variety of limits is conceivable; some may make sense, and many might have been more impartial to the outcome of the war. But there is a simplicity to “no gas” that makes it almost uniquely a focus for agreement when each side can only conjecture at what rules the other side would propose and when failure at coordination on the first try may spoil the chances for acquiescence in any limits at all.

XREF: Connects to Schelling's broader work on focal points and tacit bargaining — the idea that simple rules make natural coordination points.

Thomas C. Schelling, The Strategy of Conflict, loc. 191