Mass destruction (chemical) weapons existed and were employed in WW1. You'd expect that WW2 should have consisted mostly by aerial bombardment with much improved chemical (and other kind of horribly effective) weapons, yet what happened was an abstinence from both sides, respecting a mutual agreement of resorting to conventional weapons only. (Well, on battlefields at least, and only if discounting U.S. show off with nuclear ones at a moment when the war's conclusion was clear already.)