American POWS sent secret coded messages to Washington with news of a Soviet atrocity: In 1943 they saw rows of corpses in an advanced state of decay in the Katyn forest, on the western edge of Russia, proof that the killers could not have been the Nazis who had only recently occupied the area. The testimony about the infamous massacre of Polish officers might have lessened the tragic fate that befell Poland under the Soviets, some scholars believe. Instead, it mysteriously vanished into the heart of American power. The long-held suspicion is that President Franklin Delano Roosevelt didn't want to...