The significance of the toxic gasses' odor is not highlighted enough. The psychological effect of smell on the brain is documented and it often was the anticipation of suffering produced by the alerting odor of toxic fumes which wreaked havoc with the soldiers' psyche.
"Lieutenant Colonel S.L. Cummins, consultant pathologist with the British army in France, concluded that all divisions that were continuously exposed to chemical attack showed a significant drop in morale. The medical officer Charles Wilson was even more emphatic in ensuring that most of the men that had been gassed were frankly left in shock. By 1915, after studying its effects, the English had concluded that although they had not been designed to sow terror, the violent sensation of suffocation caused by chlorine and phosgene undermined the will of even the most determined soldiers. In fact, the mere rumor of a chemical attack even had an effect on troops that had not been previously gassed." [source]
Having being composed two years before the break of WWI, Guerlain's L'Heure Bleue is probably the most iconic perfume of the -by 1914- lost forever Belle Epoque era. The Great War saw the end of that all right.
Please read the rest of my article on Fragrantica. It revolves around the smells of warfare and associations the mind creates in times of terror with references to WWI and the Russian Revolution.