On July 10, 1941, half the residents of Jedwabne, a Polish village 85 miles northeast of Warsaw, murdered the other half. The mob, led by the mayor, were Catholics; their 1,600 victims were Jewish, slaughtered over several nightmarish hours with bats, knives, rifles and other improvised weapons. Those who survived the massacre were then rounded up in a barn donated by a local farmer, which was then set ablaze. A plaque erected at the site blamed Nazis for the massacre, but, in fact, Nazis had only authorized it. Locals walked by the plaque for half a century, knowing the truth,...