In purely military terms, the attacks of 9/11 have proven to be one of the strongest examples of asymmetrical warfare: Al Qaeda spent some 10s of millions to do $2 trillion in damage, and triggered NATO to spend trillions on wars that ended up killing more of their own people than the original 9/11 attacks did. Or to put it bluntly: the people of NATO got played
http://www.iags.org/costof911.html