I doubt you can find a thing much more expensive than war. Throughout history, wars have depleted the treasuries of countries and led to civil unrest or worse.
Do I have any historical examples in mind? Not yet. I’m going to do some research on the claim to find out if it’s true or not. I mean, one can provide a counterexample of a post-WWII boom, I suppose, but I fail to see how Iraq is stimulating our economy. Then again, weren’t we practically a command economy during WWII? I don’t know yet if my claim is true or not, but if it is, it means preventative war (and occupation) is a very dangerous strategy.
I do know, however, that war is expensive. The results of war’s expense — I will find them.