I blame the Bushes for this one.  Once his dad failed to follow through on the obvious conclusion of Gulf War One, son felt compelled to finish the job, but the arena had become a very different place and taking down Hussein was going to cost lots and lots of money in "nation rebuilding".  Estimated at $2 Trillion.  Could have been a lot of nice roads and bridges that got put off going now on two decades.
(and yes, we all know Hillary voted for this too).