Putting aside the enormous "cost" of wars in terms of human life and suffering, what about the economic costs? Once I studied the Vietnam war and Gabriel Kolko in one of his books described how much the Vietnam War cost the US in dollar terms.
So the shortlist: World War One World War Two Korean War Vietnam War Gulf War
To keep it a bit simpler, my question is: which country spent the most money on a war and which war was it? So the shortlist:
United States England Russia Japan Germany Italy
Answers can use wars and countries not in these lists; there are other candidates.