Valérie Trierweiler once called Ségolène Royal “the crazy woman of Poitou” and asked members of her partner’s communication team to keep them at arm’s length. Ms. Trierweiler is the current partner of President François Hollande, and Ms. Royal is his former partner and mother of his four children are depicted in recently published books as fierce rivals, and their animosity has been at the center of extensive media attention in France. Since 2007, when Nicolas Sarkozy took office in the middle of a marital breakdown, French politics has taken on aspects of a soap opera, complete with shifting couples...