When Sarah Palin accused journalists and pundits of "blood libel" in the wake of the deadly Arizona shootings, she reached deep into one of medieval history's most sordid chapters to make her point. The term "blood libel" is not well known, but it is highly charged — a direct reference to a time when many European Christians blamed Jews for kidnapping and murdering Christian children to obtain their blood. Jews were tortured and executed for crimes they did not commit, emblematic of anti-Semitism so virulent that some scholars recoiled Wednesday at Palin's use of the term. In a video posted...