This book has a threefold agenda: to document trends in regimes of border control; to outline the ensuing normative issues; and to sketch potential innovations to address these challenges.

Matthew Longo identifies three trends. First, he argues that states have come to see walls and fences as insufficient for securing borders. Instead, states have established zones of surveillance well before the border, relying on sensors, cameras, radar, cooperation with border communities, and inland checkpoints. We should therefore think of borders not as mere lines but as peripheral zones with a distinct institutional regime of surveillance and law enforcement. These zones raise a distinct normative problem: Longo argues that borders, by their very nature, confront state officers with unanticipated risks and threats requiring discretion in deploying violence. Following Carl Schmitt, he characterizes this discretionary violence as a suspension of the legal order, the sovereign act of “deciding on the exception.” Longo argues that because at the border “the exception arrives constantly at the doorstep” (p. 64)—that is, because of the constant scope for discretionary violence—borders are in a “perpetual state of exception” that subverts constitutional rule. The problem is that peripheral zones extend this state of exception inward into the territory of the state itself, merging military and law-enforcement functions and undermining the rule of law meant to protect against tyranny. The problem disproportionately impacts border communities, which end up dominated, always suspect, and unequal.

The second trend Longo documents—which he calls “co-bordering”—comprises jointly managed borders between neighboring states. Bilateral management includes information sharing, but also practices of “co-location” in which officers from each state act in the other’s territory. The normative significance of co-bordering is the challenge posed to sovereignty by its joint exercise over a single geographical space. The author illustrates this challenge with the “Shiprider” program between the United States and Canada, which used to enable officers from both states to patrol their maritime border areas on shared vessels. Permitting Canadian officers to engage in law enforcement in U.S. maritime jurisdictions and vice versa was, of course, illegal: Each country had to look away and not enforce its laws until a new legal framework had been developed. This is because in establishing co-bordering regimes for security’s sake, states give up part of their sovereignty. According to Longo, such erosions of sovereignty risk enabling powerful states’ domination of weaker neighbors: One of the functions of norms of territorial sovereignty, he asserts, is to protect weaker states from imperial domination.

The third trend consists in states’ increasing reliance on Big Data to guide decisions about whom to let in. This is driven by the need to combine security (to filter out threats) and efficiency (to facilitate economically significant cross-border flows). Longo extensively documents the massive data that states compile and process in order to assess the risks posed by would-be entrants, and thereby to “filter” them, subjecting “high-risk” travelers to time-consuming scrutiny but allowing “low-risk” travelers speedy entry. Some of this data is acquired at the border; some via expedited-travel or visa-waver programs; and much from private sources (Internet companies, banks, etc.) and foreign sources (via interstate data sharing).

Finally, Longo observes a growing gap between states that collect and share quality data amongst themselves and those shut out of this data-sharing “firewall.” He argues that these patterns raise three normative issues. First, by allowing decisions about entry and membership—which he deems to lie at the core of sovereignty—to be taken, not by authorized officers but by algorithms, and by relying heavily on data gleaned from outside (private and foreign) sources (whose reliability is not always transparent), the state compromises its “sovereign exclusivity.” Second, filtering people on the basis of data-driven risk assessments rather than citizenship status creates different tiers of citizens, thereby severing “the political meaning of citizenship from its legal basis” (pp. 195–96). Third, the firewall is creating a class of people, from countries without extensive data-collecting resources that are shut out of data-sharing programs, with very little data history. According to Longo, given a trade-off between rights protections and security, states increasingly condition human-rights protections on data-driven risk assessments: Those with no data history will no longer be treated as human and, hence, worthy of basic protections.

In sum, the first two trends 1) produce discretionary violence and, hence, suspend the constitutional order within the state’s territory in a way that 2) disproportionately impacts borderland populations; they also 3) erode sovereignty and, hence, risk imperial domination by powerful neighbors. The third trend 4) further erodes sovereignty, 5) creates tiers of citizens, and 6) dehumanizes dataless persons. In response, Longo proposes turning peripheral zones into jointly managed spaces and reconfiguring ports of entry as bilaterally administered by neighboring countries. He effectively envisions borderlands and ports turned into transitional zones of restricted federation between neighbors: Inhabitants of peripheral zones would be granted a cross-border “borderland citizenship” with special participatory rights in managing border policy between the two states.

Longo’s original research into recent trends is impressive, and he astutely identifies some ensuing normative challenges. But he devotes considerably less space to fleshing out his normative vision. His model of federative peripheral zones looks well suited to addressing problem number 2, by giving special voice to borderland populations, and it could assuage problem 3 if federative institutions protect weaker neighbors. But details remain murky, and his exposition leaves unclear how his proposals would address problems 1, 4, 5, or 6.

One could also raise questions about Longo’s diagnoses. Violence by state officers is not an exception confined to borderlands, nor are unanticipated risks and discretion in the application of law or violence exceptional. These are entirely unexceptional features of state power. (The author tends to conflate state-authorized violence and discretion with an “exception” surpassing the constitutional order.) Moreover, whereas in problem 1 Longo depicts individual officers’ discretion as usurping the constitutional sovereign’s authority, in problem 4 he sees an extension of sovereign authority in such discretion (and its curtailment by data-guided decisions as restricting sovereignty). When Longo argues that recent trends diminish sovereignty, he seems to have in mind a notion of absolute control that states never had: If states today filter borders using data from “nonsovereign” (private and foreign) sources, they have always relied on nonsovereign sources for deciding and undertaking “sovereign” activities (think of civilian witnesses in criminal trials, police reliance on civilian informants, armed forces’ reliance on foreign informants, Europeans’ reliance on U.S. defense, etc.). Similarly, he overplays the novelty of the tiered citizenship produced by data filtration. It is not true that states in the past “at least protected their citizens” but no longer (pp. 196–97); there have always been (not just de facto but legally sanctioned) scales of citizens with differential protections (think propertyless, women, blacks, welfare recipients, naturalized citizens, homeless, felons, etc.).

The great merit of The Politics of Borders lies in using original empirical research to introduce rich and timely conceptual innovations and to put significant normative political issues on the agenda. I only wish that the book’s exciting core were not sometimes obscured by unnecessary details and jargon. The quotations Longo includes from his extensive interviews often flesh out his narrative, but sometimes we could have been spared their reproduction (e.g., p. 208: “As one data analyst explains: ‘Governments of neighboring countries need to agree on certain standards, certain procedures, certain ways of doing things’”). The author takes obvious delight in introducing new vocabulary, and sometimes he hits the mark (e.g., “co-bordering”), but often the jargon is unnecessary (e.g., “spectral sovereignty” for the banal idea of degrees of sovereignty); and sometimes the fancy language distracts (e.g. “normatively affirming” to mean desirable or good [p. 130]). These qualms notwithstanding, by drawing our attention to the normative consequences flowing out of peripheral zones, co-bordering, and Big-Data border filtering, Longo has done a tremendous service to border studies.