In 1919 the vast stores of British & American produced poison gases are used in indiscriminate bombing of the Rhineland to support the Allied advance. Tens of thousands die of gas, whilst those who survive are weakened and millions more succumb to the Influenza epidemic.
I had a very similar idea - August 1918 - The Germans decide that they will use mustard gas, dropped from Zeppelins in night raids over England. The targets are supposed to be the various military camps around Surrey, however due to a mix of unpredicted winds and a colossal navigational mess-up, the Zeppelin finds itself over London. After preparing to return to Germany, it is realised that some of the cannisters have begun to leak, so the decision is made to dump the cannisters overboard thinking they are in relatively unpopulated Surrey.
As the chemicals hit the London streets, basement windows are open where people take shelter, and the heavier than air chemicals seep down, ever downwards and in one case, to the entrance of a tube station sheltering hundreds of families.
As the gas entered the tube station, the initial injuries from the gas were comparatively minor, however ten times that number were injured in the ensuing panic while 17 died of electrocution after being pushed onto the rail.
The response from London is immediate, the following evening, bombers from the newly formed Royal Air Force, Launched from France drop mustard gas bombs on Stuttgart in a deliberate reprisal raid, the death toll is barely over a hundred, but the casualties rate in the thousands.
This sets the standard for the remains of the war, London is hit three more times, however the installations of gas curtains in tube stations lessens the impact to negligible by the third raid. Dover is hit 4 times, along with Paris 8 times. In retaliation, German cities are also hit time and time again until the eventual armistice in November 1918. With the general health of the population as low as it was, the Spanish Flu outbreak of 1918 decimated the population of Western Europe and America, with literally % of the population dying as a direct result of the outbreak, with famine stalking Germany, Belgium and France, with refugees flooding into neutral Netherlands, the total deaths arising directly from the war and the effects of it was 24.6 Million people, nearly 10% of Europe's prewar population.