Peter Martins, who picked up the mantle from the famed choreographer George Balanchine to lead New York City Ballet over the last three decades, is taking a leave of absence in the wake of a sexual harassment accusation, the company confirmed on Thursday night. City Ballet and its School of American Ballet had started an investigation into Mr. Martins, 71, after receiving the accusation of past harassment, which was made in an anonymous letter. The two organizations jointly retained Barbara Hoey, the chairwoman of the law firm Kelley Drye’s labor and employment practice group, to conduct the inquiry. The specifics...