The 20^th century ended with two great utopias, the end of history and a borderless world, but the 21^st century -for many starting on 11 September 2001- ended with them. With the various processes re-bordering, such as the creation of the Agency for External Borders of the European Union in 2002, the construction of the border wall US-Mexico in 2006 (and again proposed in 2017) and in India with Bangladesh and Burma since 2003, or the use and creation of biometric massive databases ports of entry, among many others, it became clear that far from disappearing borders were strengthened and multiplying.

Since the emerging critical border studies, the attention has increased to the multiplication and tightening of border regimes, with a bigger emphasis on the conceptualization of the border as a practice, which is produced and reproduced in many ways, for many actors and at any place and time. A good part of these studies have had a clear foucauldian orientation and, generally, of the critical theory. And this is no exception of the work of Thomas Nail, associate professor of philosophy at the University of Denver, an atypical profile in border studies, where geographers, sociologists, anthropologists and political scientists stand out. Likewise, the title, Theory of the border, refers to one of the main debates in this field: is it possible (or desirable) a general theory of borders? In the past 20 years it has been discussed widely, even with no conclusive answer since the border is still poorly understood, and as it becomes more complex, it is more difficult to define. In this direction, Nail provides a systematic approach covering territorial boundaries, legal, political and economic, with the aim of developing theoretical framework that “will allow us to understand the history (not Idealist) conditions in which empirical borders emerge across different social contexts” (p.12, emphasis in original).

This work is part of a much larger research project on kinopolitics, the theory of the movement of flows. In 2015, Nail published a first book, The figure of the migrant (^{Nail, 2015}), which was followed by Theory of the border and, as announced on his website, at least three more. This research focuses on the nature of borders as a result of kinopolitics processes of expansion and expulsion. The border is defined as “a process of social division” (p.3) rather than as a product of that; for example, the border is not the result of a state or society, but on the contrary, those are the result of bordering. In addition, while division process, the border is not a technology impediment of flows, but bifurcation, redirection and recirculation of those.

The book is divided into three parts (plus an introduction and conclusions): first a theoretical and conceptual construction of kinetic functions of the border (I), followed by a historical exhibition on the emergency of four border regimes (the fence, the wall, the cell and the checkpoint), in which some or other functions predominate (II) and, finally, its application in a contemporary border, US-Mexico, where the four regimes appear combined and hybridized (III).

The first part (Chapter 1) is very abstract, and for further development the author refers to the above explained in The figure of the migrant. Defends a theory of border is in fact a theory of movement (kinopolitics) in which the central concepts are flow, node and circulation. While the first one refers to a continuous movement, the nodes are points of relative stability in that movement, and the circulation is the movement regulation. In this context, the border is a node, which identifies four kinetic functions: offense, defense, binding and kilter.

In the second part (Chapters 2-6), Nail defines the fence as holes in the ground and the vertical objects that fill them, whose emergency dates back about 10 000 years BCE. It has a centripetal function, while redirecting flows from the periphery to the center, but do not require a centralized power. Under this regime it includes various types (and subtypes) of technologies, such as stockyards, stockades and monuments. For example, some of these first fences were the stockyards of the Neolithic, which funneled the prey toward the hunters without them noticing. The fence provided the conditions that later would bring the centralization, and the wall that was emerging during urban revolution (toward 3 000 BCE). The wall is defined as a set of stacked bricks, where the brick is any material produced in a standardized manner, including people. In this case its function is centrifugal, to expand the center to the periphery, and the walls include not only the barriers but also soldiers, military grids, roads and ports: offer defense to cities and at the same time redirect resources to the conquest and trading. The third regime, the cell, emerged in Europe in the Middle Ages, and is the tension between dividing and linking individuals. In this case the rooms of monasteries, prisons and hospitals are included, as well as guidance letters, passports, calendars and schedules. All have in common the establishment of identification and jurisdiction. Finally, the checkpoint emerged and consolidated during the Modern and Contemporary ages, and represents the elasticity, which allows inspection at anyplace and anytime. This includes police patrols, spies, private property and the nation as well as informational places where are collected, sorted and analyzed individual data.

Finally, in the third part (chapters 7-10), it classifies the technologies used in the US-Mexico border according to the four previously identified regimes that currently appear simultaneously and in combination. With few exceptions, the cases correspond to the United States, which makes it impossible to identify how the border regimes of that country interact with Mexico; likewise, it focuses on undocumented migration, leaving aside the documented and other flows as the commercial (legal and illegal). In any case, the analysis clearly shows that the border not only necessary to include and exclude, but also redirects flows (such as detention and deportation of migrants), while it can appear anyplace, anytime. Nail distinguish some interesting analogies to deepen; for example, the border fence would work the same way Neolithic hunting stockyards, in one hand undocumented migrants and in the other animals. However, on several occasions you can sense that the identification of all types (and subtypes) border (and each of the regimes) is forced. Similarly, several of the results are independent of the theory developed in the two preceding parts.

In fact, Nail does not build a theory of the border, because he does not offer explanations or predictions. In the contrary, it provides a taxonomy of different types and subtypes of frontier technologies; but no explanation of why these technologies emerge, what motivate them, who are heading to or how they combine. These limitations are especially evident, as already noted above, in the case of the study analysis. While taxonomy, this work is part of what has been one of the main lines of border studies from the 19^th century, description and classification of types of borders. A recent example of this is found in the article by ^{Benedetti (2014)}. A second limitation is the absence of other actors than the state in the practice of the border. This is surprising, since in the book the author insists that the borders are not limited only the states, as it was in the classical political geography, or that borders are the result of negotiation between states and state actors. In the third part this translates into the analysis of just state actors in the US, but not in Mexico or non-state actors such as border monitors, drug cartels, smugglers, activists of human rights or border population. This absence is due in part to the author's decision to leave out of the book the resistance and subversion of the border, as announced in the first pages and, perhaps, will be addressed in the following books. All this, however, does not detract importance and validity to this work; on the contrary, it is an original and inspiring approach that opens up many possibilities for the future development of critical border studies.