There are many things to be said about the 1938 analogy of which the neocons and their fellow travellers are so fond. (You can see it again today here and here and here and here.) Beyond the obvious stupidity of the fact that most of those who invoke this metaphor have little idea of what actually took place when Neville Chamberlain met with Hitler in Munich of that year, let me address some of the embedded assumptions that almost always are present when people say that some contemporary crisis is "like 1938," or when they suggest that someone who is taking a dovish line is being an "appeaser" or "like Chamberlain."
First, the analogy implies that those advocate a tempered line with respect to some current crisis lack "moral clarity." It suggests that they fail to appreciate that it is simply unacceptable that a nefarious Great Power be allowed to bully a small, innocent country that is clamoring for others to help her. Second, it suggests that a given dovish line is not only knavish, but also foolish, for it fails to realize the truly nasty nature of the opponent in question.
Before I unpack why this historical metaphor is more complex than it seems, let me begin by stating the obvious, namely that there is no question that Chamberlain was a fool. He was a fool because he thought that Hitler was someone one could credibly "do business" with, and he was a fool for proclaiming that the agreement he had made with Hitler would bring "peace in our time." What he was not a fool for, however, was in thinking that the fate of Czechoslovakia was of no real importance to Britain, not worth going to war over. Some context here, is of course crucial: Chamberlain's meeting with Hitler was less than two decades removed from the Great War, a war that largely destroyed European civilization because the Great Powers decided to fight each other to the death over the fate of some pissant country. No one wanted to make that mistake again, which was why many congratulated Chamberlain for steering clear of war.
But the third assumption embedded in the 1938 analogy, which is not just the least visible but also the most important and above all the most questionable, is an implicit counterfactual, namely that if only Chamberlain had taken a harder line with Hitler, then either Hitler would have backed down and the eventual war could have been avoided, or else Hitler would have been forced to fight then, when he was less prepared. But of course, because it's a counterfactual, we don't actually know if that's true -- it's unknowable, despite the assumptions of those who invoke the metaphor. As Bruce Kuklick has pointed out, "To identify appropriate structural features of an event as evidence of bullying [implies] that standing up to the bully would bring success. But we do not know what would have happened at Munich had Chamberlain been forceful.... Had the English faced down Hitler in 1938, he might have bided his time and become far stronger when he went to war later" (Blind Oracles, p. 122). Had Hitler bided his time to build his military strength, he might have defeated the Russians in his initial push, in which case he might well have won the war.
This is the key thing that the invokers of "the lessons of the 1930s" fail to recognize: that different, more aggressive choices by the Western powers might not have led to a better outcome. They assume it would have, but they don't know that. They can't know that.