In recent decades, the relationship between Argentina and Brazil was understood as an alliance sustained by common interests and shared values. We argue that this assumption no longer holds, and that the relationship is going through a growing divide. In this article, we examine the evolution of Argentina–Brazil relations and carry out a plausibility probe to test three explanations of their rise and decline. To do so, we look at the shifting patterns of preferences (i.e., domestic coalitions and the electoral cycle), interests (i.e., business actors and trade patterns), and the shadow of the future (i.e., prospects for future interactions). We find that each of the three explanations contributes significantly to our understanding of the relationship's development and the challenges that lie ahead.