"...what did you mean? I thought the whole point was that Saddam presented a danger to us in the future, and we invaded to preempt his future actions. "

The preemption doctrine was/is an attempt to change/shape international law.
Whereas the invasion of Iraq was in accordance with existing international law.
To think that the invasion of Iraq was an application of preemption doctrine is [in my view] a dangerous misunderstanding of what is meant by the preemption doctrine. More discussion is needed on preemption doctrine- we want to be able to as much as is possible to stop terrorists from being able to kill tens of thousands of people using weapons of mass destruction.