That's exactly my point. It would have been better to spend large amounts of money preventing 9/11 from happening so we wouldn't have spent the 5 trillion we did on our reaction to it (never mind the cost it had to our national psyche). If someone hits a skyscraper in NYC or the Pentagon with what is effectively a cruise missile, the US military is going to be on the ground to get the bad guys within the month and we'll stay until we find them. That's just a law of nature; its what the people demand from their government, not just in America but in general. Given that, it makes a lot of sense to spend money on preventing terrorism. The direct and indirect costs of a successful terrorist attack are huge.