Here’s a passage from page 18 of volume 1 of Roger Backhouse’s forthcoming (in May) biography of Paul Samuelson (1915-2009): Founder of Modern Economics: Paul A. Samuelson:
His remark that he could see the Keynesian multiplier at work when he was on the farm [that he often visited as a boy] is clearly a later interpretation, but he claims that he was aware at an early age of wartime prosperity due to high grain and steel prices, and of the sharp post-war recession that preceded the longer boom of the 1920s.
Samuelson here, like many noneconomists, fell victim to the fallacy of composition. Wars that make certain farmers and industrialists (and their workers and suppliers) more prosperous do not thereby make society more prosperous. What is true for some in this case is emphatically not true for the group.
The diversion during WWI of resources, including manpower – much of which was literally slaughtered on European battlefields – made Americans (and, of course, also Europeans) at large more impoverished rather than more prosperous. This reality binds even if it is the case that the war was necessary, noble, and conducted as efficiently as possible.
Butter and plowshares converted into guns makes the bulk of the population poorer, not richer. There is no such thing, for society writ large, as wartime prosperity. There is only wartime impoverishment.