Paul Krugman pounds his head against the wall:
Spending in wartime: One of the small compensating benefits of the economic crisis is that people have suddenly realized that economic history is relevant. Unfortunately, some of the attempts to use that history are spectacularly off-base — such as the attempts by conservative economists to use experience during World War II to argue that the multiplier on government spending is low. I’ve written about this here and here. But I thought a bit more data might be instructive. You see, Robert Barro made much of the fact that private spending actually went down during World War II — which he took as evidence of “crowding out”. But what types of private spending fell, and why?...
[S]pending on new homes and cars before, during, and after the war years. Both basically collapsed. Why? The answer is that (1) There were draconian building restrictions in effect — in fact, the end of those restrictions helped set off the postwar housing boom, and (2) new cars weren’t being produced, because the factories were making tanks instead (and if you did manage to acquire a car somehow, gasoline was rationed).
Why anyone thinks that private spending during those years is a model for what will happen as a result of fiscal stimulus now is beyond me.