Seventy-five years ago, on Sept. 30, 1938, British Prime Minister Neville Chamberlain signed the Munich Pact, handing portions of Czechoslovakia to Adolf Hitler's Germany. Chamberlain returned to Britain to popular acclaim, declaring that he had secured "peace for our time." Today the prime minister is generally portrayed as a foolish man who was wrong to try to "appease" Hitler—a cautionary tale for any leader silly enough to prefer negotiation to confrontation.

But among historians, that view changed in the late 1950s, when the British government began making Chamberlain-era records available to researchers. "The result of this was the discovery of all sorts of factors that narrowed the options of the British government in general and narrowed the options of Neville Chamberlain in particular," explains David Dutton, a British historian who wrote a recent biography of the prime minister. "The evidence was so overwhelming," he says, that many historians came to believe that Chamberlain "couldn't do anything other than what he did" at Munich. Over time, Dutton says, "the weight of the historiography began to shift to a much more sympathetic appreciation" of Chamberlain....