Historically, Mexico, Central America, and the United States make up a triangular system that is extremely important for the isthmus. The triangular methodological perspective allows us to highlight some important differences between Mexican and United States conduct toward Central America. The United States occupies a privileged angle of influence over Mexico and Central America, because of its position as a world power and its historical inclusion of Central America and Mexico within what it considers its sphere of influence. Mexico's relations with Central America were strongly affected by its relationship with the United States, which has always considered Mexico part of its "sphere of influence." Mexico and Contadora offset the United States presence in Central America and opened up some room for the exercise of national autonomy. The chapter also presents an overview of the key concepts discussed in this book.