Here's an interesting post at The Sophistry blog about the issue of preemption. It raises some interesting points, not all of which I agree with, and is in general an interesting read. Of course, I'm all for preemptive strikes against our enemies, and agree with elements of the linked discussion that the concept of "preemption" has a pretty broad timeline.
I won't toss in more examples, because the linked post provides a few. Give it a read.