A former Japanese diplomat confessed to taking part in the 1895 assassination of Korean Empress Myeongseong, according to a Korean historian who examined recently discovered letters that provide a fresh account of the brutal slaying. Also known as Queen Min, Myeongseong was the wife of Gojong, the 26th king of Joseon, a Korean dynastic kingdom, and was known as an influential figure in the palace. In one of the letters, the diplomat proclaimed, “We killed the queen,” and went on to detail the circumstances surrounding the assassination. The slaying, known as “the Eulmi Incident” in Korea, involved a group of...