Something of a thought exercise here, and Yorick there is no need for another "all wars are bad" speech -- I agree already
Of all the wars in the past, which is the one you would consider the worst to have been stuck in as a participant?
I would say World War I, specifically on the Western Front. Paraphrasing Ernest Hemingway, there has never been such dreadful, colossally mismanaged butchery on the face of this earth as occurred in the 4 years of that war.