I'll disregard the Hitler reference because that doesn't serve to advance debate. It probably wouldn't be too hard to find several non-Nazi references to preemptive attack, such as Israel's attack that destroyed the Egyptian Air Forces back in 1967.

Instead of throwing heated political quotes back and forth, here's the definition of "preemptive attack" from Joint Publication 1-03 DoD Dictionary of Military and Related Terms:

preemptive attack — An attack initiated on the basis of incontrovertible evidence that an enemy attack is imminent.

Now, if you do indeed have evidence that an enemy attack is imminent, what should you do? Do you sit back and take the attack before reacting or should you try to stop the attack? If you choose to do nothing, how do you answer the question "Why didn't you do something?" to the survivors and the victims' families?

To put it on a more personal level, if you saw someone drawing up to throw a punch at your head, do you let him strike you or would you try to block the punch?