Imagine a wealthy person finds, one morning, that his house has been vandalized, a stone thrown through one of the windows. While, yes, it does trigger economic activity for the person to hire a glazer to come fix the window, you only see that with a very simplistic model. For surely, the homeowner is not simply choosing to either fix a window or not, throwing the cash away without spending it on anything. The homeowner must now choose between buying something else (let's say, a pair of shoes). Without the stone, it is possible to have both a perfectly sound household AND a pair of shoes; with the stone, we must choose between a good window OR a new pair of shoes (or some other quantity of goods).
The so-called Broken Window fallacy follows WWII arguments like a vengeful ghost. The fact is, if you DON'T bomb a city, and instead use reconstruction funds for, say, construction funds, you can end up with TWO cities. Or maybe one city that is twice as wealthy, healthy, and clean.
We cannot say that "without WWII, the economy wouldn't have rebounded". After all, the economic conditions of the great depression LEAD to WWII, as they lead to facism and nationalism in various mediterranian, greco-roman countries which will remain nameless in the current recession. Furthermore, I'm not saying "without WWII, the econonmy would have rebounded on its own", because there's even less historical evidence to argue over in that case (there not being any alternate earths to access where WWII didn't happen).
If there is one thing to take from George Orwell's 1984- one thing he did get right right off the bat- it is that war is a useless consumer. I would add that even worse, it has an addictive effect to the economy. Imagine that we were to stop the wars, draw down and fire the troops, and immediately stop manufacturing the majority of war materiel. Such a thing would be a serious blow to the economy.
Truly, wr are damned if we don't, damned if we do, damned if we stay here, just plain damned.