LAST month, I received a startling email from an employee at one of Japan’s largest newspapers, about a development I’d long awaited. The government was about to unveil a 12,000-page, 61-volume official biography of Emperor Hirohito, which a large team of scholars and civil servants had been preparing since 1990, the year after his death.

I was asked if I would examine an embargoed excerpt from this enormous trove and then comment on the emperor’s perspective on various events, including Japan’s 1937 expansion of its conflict in China and its decision four years later to go to war with the United States and Britain; the trial of war criminals; the atomic bombings of Hiroshima and Nagasaki; and the American military occupation of postwar Japan.

But there was a condition: I could not discuss Hirohito’s “role and responsibility” in World War II, which would be strictly outside the scope of the newspaper’s reporting. Having devoted years of my life to examining precisely this topic, I politely refused.

The release of Hirohito’s official biography should be an occasion for reflection around the world on a war that, in the Pacific theater, took the lives of at least 20 million Asians (including more than three million Japanese) and more than 100,000 citizens of the Western Allied nations, primarily the United States and Britain.

Instead, Japan’s Imperial Household Agency, abetted by the Japanese media, has dodged important questions about events before, during and after the war. The new history perpetuates the false but persistent image — endorsed by the Allied military occupation, led by Gen. Douglas MacArthur — of a benign, passive figurehead...