Mustard Gas Attack

Because the date in the OP is 1917 not during WW2

*Facepalm* Sorry about that, I don't know how I could have missed that.

In answer to the OP
I am not sure the airplanes of this time could actually reach London and efficiently deliver enough gas to poison a significant area, but assuming it is possible and it would be a blow to British morale. However the Kaiser would skin alive whoever targeted the king (he was a cousin of the British Royal- ie. his grandmother was Victoria). Specifically targeting a fellow monarch (and a family member) in a civilian center would have been totally off-limits in the Kaiser's view.
 
Well if a Zeppelin was used to gas London during WW1, I can see a lot more repercussions being extracted from Germany. It might even bring about War Crime Trials against the officers involved. Maybe even up to Kaiser Willy.
 
Top