The Israeli prime minister is invoking the lessons of history to make the strongest possible case against Iran, even if that means deliberately overstating the putative equivalency between that country and Nazi Germany. With President Mahmoud Ahmadinejad's regime steadily moving closer to acquiring nuclear weapons while continuing to encourage its followers to chant "Death to Israel," Netanyahu can hardly be blamed for taking those threats seriously.

But what are the real lessons of history -- and what do they tell us about how we need to conduct ourselves today?

On that score, there's strong supporting evidence for Netanyahu's broader point about the dangers of underestimating the threat from regimes spouting radical rhetoric, but less than convincing evidence that history offers a clear guide to what constitutes a sensible course of action.

Although it seems incredible now, many people initially saw Hitler as a bizarre, effeminate politician who would never be in a position to inflict real harm -- or, later, as a pragmatic leader we could deal with.

This was true not just of the British and French leaders who signed the infamous Munich Pact of 1938, which led to the dismemberment of Czechoslovakia. As I point out in my new bookHitlerland: American Eyewitnesses to the Nazi Rise to Power, it was also true of many Americans who lived and worked in Germany.

Dorothy Thompson, America's most famous woman foreign correspondent of that era, interviewed Hitler in November 1931, fourteen months before he became chancellor. She entered the room expecting to meet the future dictator of Germany, but "in something less than fifty seconds I was quite sure I was not," she wrote. Struck by the "startling insignificance of the man" who is "inconsequent and voluble, ill-poised, insecure," she predicted: "If Hitler comes into power, he will smite only the weakest of his enemies."

German politicians often made the same mistake. Franz von Papen, the vice chancellor who helped engineer Hitler's appointment to the top job, told his friends: "We have hired Hitler" -- in other words, he would be easily manipulated.

In many cases, even German Jews refused to take Hitler seriously. Paul Drey, a Bavarian from a distinguished Bavarian Jewish family who worked for the U.S. Consulate in Munich, wrote off the Nazis' early successes as "a temporary madness," insisting that Germans were "too intelligent to be taken in by such scamps." Drey would die in Dachau.

To be sure, there were those who sensed Hitler's dangerous potential right from the start. Captain Truman Smith, a junior U.S. military attaché, first met the little known Nazi leader in 1922, immediately warning that he was "a marvelous demagogue" who could go far. And along with many of her journalistic and diplomatic colleagues, Thompson radically revised her view of Hitler as soon as he seized dictatorial powers.

Still, when it came to resisting Hitler's expansionist aims, there was plenty of disagreement. Perceptive journalists like William Shirer of CBS despaired that visitors from Paris, London and New York took at face value Hitler's protestations that his intentions were peaceful. "Peace?" he wrote in his diary in 1937. "Read Mein Kampf, brothers."

But most outsiders didn't read Mein Kampf, and even among those who did there was no consensus on whether its vitriolic attacks on Jews, democracy and bolshevism, along with Hitler's stated ambitions to conquer vast territories in the east, should be taken literally or viewed as merely a cynical electoral ploy.

All of which, Netanyahu argues, stands as proof that the greatest danger is to discount the new threats of our era. But 1938 has been invoked before as justification for military action, at times with tragic results. As President Lyndon Johnson escalated the war in Vietnam, he claimed that he was seeking to avert another Munich. To this day, the country is split over whether the ensuing loss of American lives and treasure was justified at any point or a disaster from start to finish.

It isn't easy to determine which situations demand the kind of forceful action to stop a potential aggressor that was so woefully lacking in the 1930s. Netanyahu is right that history teaches us that we ignore the fiery rhetoric of radical regimes at our own peril. Unfortunately, though, history -- especially the history of the Nazi era -- doesn't offer many immediate lessons beyond that.

It certainly doesn't tell us what we really want to know: whether we are making the same mistake today with Iran -- or is the situation so different that a bigger mistake would be to overreact.

Andrew Nagorski, vice president and director of public policy at the EastWest Institute, is author of the forthcoming Hitlerland: American Eyewitnesses to the Nazi Rise to Power.

Click here to visit his website.

Click here to read this piece in The Huffington Post.