[PAR35]

But it was also the only poison gas incident of World War II

I'm not sure about that one. I guess it depends on what one means by "World War II". The Italians used gas against Ethiopia, and I think the Japanese used it in China.

[SAMWolf]

Ony if you count the "offical" start of WWII as Sep. 1st 1939. ;-)

[Cheburashka]

I also recall a claim that the Germans used poison gas in a limited manner during the siege of Sevastopol in 1942 - probably to clear underground bunkers. I don't recall any details.