As both a candidate and now as president of the United States, Donald Trump has frequently reiterated his desire to dam the flow of immigration by building an impregnable wall, declaring “no border, no country.” In Theory of the Border, Thomas Nail takes a more nuanced view of the relationship between borders and modern nation-states, though he agrees with Trump that the border is foundational for nation-building. Whereas Trump deems the border a necessary condition of existence for America, Nail treats borders as the condition of existence for all social mobility and politics in general. He articulates a political theory based on human movement that he dubs “kinopolitics.” Nail sees the border not as a set of stable lines on a map, physically separating two groups, but as a ubiquitous process of social division that redirects flows of movement. Today, borders are not confined to the barbed wire structures that divide states; rather, they structure the process of dividing social groups, organizing everything from passport controls to the singing of national anthems.

Nail traces the evolution of bordering mechanisms through history, beginning with the emergence of fences corralling animals in the Fertile Crescent and ending with the refined checkpoints designed to regulate movement between nation-states. He then applies theory to practice by examining the U.S.-Mexico border. The theoretical legwork occupying the first half of the book allows Nail to unravel, succinctly and in detail, the multifaceted dimensions of one of the most controversial borders in the world today. In the end, Nail demonstrates how border technologies produce and constantly reproduce the contours of the societies they enclose.

Understanding the logic of the border is crucial for dealing with immigration policy, of course, but Theory of the Border transcends the scope of that issue, concerning itself with nothing less than the central contradiction of globalization: income inequality continues to grow even as more people are migrating now than at any other time in history. Transnational institutions like the EU have not eradicated borders. Globalization has merely dislodged borders, moving them from their traditional perch at the frontiers of civilization to the center of the social space. Today borders are proliferating, not disappearing. Donald Trump hyperbolizes when he demands a “secure” border, since borders can only redirect movement, not eliminate it. But while his project may be impossible, Trump does stumble upon an important truth about borders: In one way or another they underlie almost every important political issue today, from income inequality to political identification.