In the summer of 1968, Mary Soderstrom and her husband loaded up their VW Beetle and immigrated to Canada from the United States. The contrast between their new home and their old led to a long-running reflection on what makes the two countries different. How could two places that are similar in so many ways be so disparate in others? In Frenemy Nations, Soderstrom answers this question by addressing a range of geographical "odd couples": including the United States and Canada; New Hampshire and Vermont; Alberta and Saskatchewan; Haiti and the Dominican Republic; Scotland and Ireland; Rwanda and Burundi; and more. Through it all, Soderstrom shows how tiny differences--in geographic features, colonial histories, resource competition, education, women's roles, language, and migration--can have outsized effects on how polities develop.