Comment by VLM

Its the classic logistics problem. Scaling ratio of weight vs volume or something like that. Just like nukes, if you heat an enemy soldier to 100M degrees he isn't any more dead than heating him to 10M degrees and volumes expand very slowly with mass so making bigger and bigger bombs is a fools-errand.

Same problem with chem weapons. You hit a tank brigade with 1000x lethal dose they aren't any deader than if you hit them with 1x dose. But if the bomb misses which is likely, all you've done is REALLY piss them off. Nerve gas in an empty wheat field just kills a bunch of corn bugs but it really pisses people off. If you target their tank brigade and miss, they'll target your home town, as we did to them with conventional bomb even without having been nerve gassed to start with. If you target their home town then the brigade you missed is going to be unhurt and really angry. Its the kind of weapon thats pretty useless unless you have infinite supply and infinite logistics. Like cold war USA or cold war Russia.

The allies had better logistics than the Germans so they knew the second time around in WWII that trying to go chem is just going to end up in the German's getting more chem'd than the allies.

Another issue is WWI and previous its all about siege warfare and breaking sieges where WMD is awesome and useful, whereas WWII and newer is all about maneuver warfare and blitzkrieg and all of Germany's plans and all of their early success were based on the idea that anything in range of shells or aircraft today is going to be occupied rear supply area next week at the latest, so destroying it would be pretty dumb because we need that area to be the rear of the battle space next week. For a modern comparison the USA could have nuked the green zone in Iraq and there's absolutely nothing anyone could have done about it, but 'we' knew we'd be occupying the green zone and needing something like the green zone, and the green zone is sitting there for the taking, so in an incredibly short term perspective it would have saved troops and saved time and saved effort to just nuke it instead of taking it the old fashioned way, but in medium and longer term it would be counterproductive to war efforts to use WMDs against the green zone, so we didn't.