Churchill and others were fully aware of the danger posed by the Nazis during a time frame when it would have been possible to take them out. At that time the Nazis simply did not have the military might to resist a preemptive attack.I think the comparison between Iran and Hitler's Germany is at best very shaky and at worst entirely specious.
Had that occurred, it is very likely it would have saved something on the order of 40,000,000 deaths. Almost certainly tens of millions.
Is the moral principle you stated above so high and pure that you would choose to have those deaths occur again in order to follow it?
The geopolitical situations are completely dissimilar. Even the most tinfoil-hat paranoid evaluation cannot elevate the threat of 2007 Iran to anything even comparable to the threat of 1939 Nazi Germany.
There were ample opportunities to confront Germany after it had violated moral standards and justified war. The notable historical event here is Chamberlain's acceptance of the invasion of Czechoslovakia, which would justify war under the principle of self-defense.
There are other possible justifications for war which I omitted, such as the mass murder of the Jews, which do not at all apply to the situation about Iran.
Most importantly, it was and is not possible to know in advance the consequences of action or inaction. Hindsight is 20/20--but if we're permitted to go back in time, we're better off addressing some of the more fundamental causes of Hitler's Germany, notably Germany's economic fragility due to its ruinous WWI reparations.
We could likewise say that we could have prevented Columbine massacre by simply assassinating Eric Harris and Dylan Klebold at the first sign of trouble. Would arguing against summarily executing children who might pose such a threat indicate any approval or toleration of their actions? I think not.
You can rationalize anything with such an ex post facto analysis. But in the real world, we cannot see very far or very well into the future; we can make our moral decisions only on what we know of the immediate present.