Neal, if you study your World War 2 history carefully, you'll realize that the Allies simply could not have nipped Hitler in the butt in the 1930's.
Following the first World War, nearly all the countries of Europe, but especially Britain and France drastically downsized their armed forces. Trench warfare was so unbelievably horrifying, that the people of Europe never wanted to fight again.
In the treaty of Versailles, however, the Allies really gave Germany the shaft, which pissed off a lot of Germans, making it easy for the Nazis to rally support in the late twenties, early thirties. Britain and France decided to ignore Hitler's march into the Rhineland, not because they didn't have a preemptive doctrine, but rather because they were absolutely terrified of starting another war, and because they could not have sustained an attack against Germany at the time.
The difference between then and now is that we took Baghdad in 3 weeks, and we knew that we could. We planned on doing it that way. In 1934, Britain and France were not entirely sure they were even capable of taking Berlin, let alone on any sort of quick timetable.
Anyway, I'm really digressing here. Overall I think preemptive warfare is acceptable only in the most extreme of situations (like Kim Jong-Il goes on TV, points to the camera, and says, "Hey America, I'm going to bomb you."). Bush's reasoning for the war in Iraq is extremely questionable at best.