Things you thought you knew, that just weren't so: British use of mustard gas in the Iraq Revolt of 1920.  I had read about this ten-fifteen years ago, and just assumed it was substantiated.  After all, mustard gas was part of all major armies' inventory right through World War II - an Allied supply stock of the stuff in Bari during the Italian campaign was smashed open in a German air strike and the cloud killed hundreds.  And the British were pretty open about their legal opinion that the Geneva Accords only applied to "civilized" opponents, and had just spent four years gassing and being gassed by the nominally civilized Germans.  They had even used chemical weapons in Palestine a few years previously.  And both the Spanish and the Italians used chemical weapons against North Africans and Ethiopians between the wars, so it's not as if this was unthinkable at the time... just maybe not actually true.