I have a few points to make here: World War II did have gas. Japan used chemical weapons in China. Setting that aside for a moment, the various theaters of wa

I have a few points to make here: World War II did have gas. Japan used chemical weapons in China. Setting that aside for a moment, the various theaters of war did not all immediately escalate to total war all at once. Rules of war were initially followed but selectively abandoned over time when one side or the other also abandoned either the same rule or a complementary rule. For example, consider treatment of POW’s. There are complementary international laws at work here: it’s a war crime to kill a surrendering enemy rather than take them prisoner, but it’s also a war crime (perfidy) to falsely surrender only as a ruse to kill your would-be captors. After running into enough Japanese perfidy (and very little genuine Japanese surrender), American troops in the Pacific stopped bothering with taking prisoners. This was dramatically different from what would happen in theatres such as North Africa. You also mention strategic bombing, but that was also a story of tit-for-tat escalation, aided along by genuine targeting mishaps. The Germans notably refrained from civilian targets during the early Battle of Britain, only to embrace terror bombing during the Blitz. This escalation was supposedly a response to some British bombing raids that, supposedly by accident, hit civilian targets in Germany. When it comes to gas, the reason it wasn’t used outside of China was because no one had an incentive to escalate to that particular measure. Germany didn’t use gas because if they did, the retaliation would have devastated them—German logistics were overwhelmingly horse-centric and Germany did not have anywhere near enough gas masks for their horses. The Soviets didn’t use gas because Germany had a well known advantage in chemical warfare and they didn’t want to invite Germany to retaliate with gas. The western Allies didn’t use gas because they, unlike the Axis and Soviets, did not generally violate the laws of war unless the enemy violated them first. As another general point, many of the war crimes committed in World War II were purely evil to the point of counterproductivity. For instance, mistreating and murdering prisoners of war eliminates any incentive for the enemy to surrender, while terror bombing usually makes enemy populations more rather than less motivated to continue supporting the war effort.