Americans have always had mixed feelings about history. The Founders were students of history, including ancient history. Indeed, they believed that the history of the Roman Republic shed as much light on their enterprise as did the history of England. They considered King George III a tyrant in the mold of Julius Caesar. Ever since, in every generation some Americans have criticized one U.S. President or another as a new Caesar. Such sloganeering, however, does not mean that the study of history is in the national DNA. The Founders’ classical education was soon as out of date as their powdered wigs.

When Harvard philosopher George Santayana said in 1905, “Those who cannot remember the past are condemned to repeat it,” he presumably meant this as a warning to those who did in fact not study history. If so, he had no shortage of examples. His contemporary Henry Ford put it bluntly about a decade later: “History is more or less bunk.” Ford went on to say: “We don’t want tradition. We want to live in the present, and the only history that is worth a tinker’s dam is the history that we make today.”

That same year, 1916, a national educational association recommended that American high-school curricula offer less history and more of the new field of social studies in order to promote what they called “social efficiency.” A century later, our don’t-know-much-about-history society is the result. Or was it already there? True, in 2017 it was noted that half a dozen recent national surveys portrayed a declining knowledge of history and civics. Yet as Diane Ravitch once put it, there was probably never a golden age when students were knowledgeable about American history. Our national character is just too pragmatic and forward-looking for that. One sign of the times: History continues to flourish in American universities, but with an increasing bent toward the modern period. There are some important and honorable exceptions (turning to the social sciences, think, for example, of the work of Douglass North and the new institutional economics), but there is little doubt about the general trend.

Into our historically challenged society comes Jakub J. Grygiel with an opportune reminder that premodern history still matters. In Return of the Barbarians he offers parallels and comparisons between the barbarian challenge that ultimately brought down the Roman Empire in the West and the threat of non-state actors today. As a political scientist with a deep knowledge of history, he is an ideal practitioner of comparative analysis. As one who has served in the U.S. Department of State, Grygiel is well informed on contemporary international affairs. And he offers an impressively wide perspective through a series of case studies. With a look back to the Greeks, the author focuses on the Romans and then takes the story forward to the 17^th century and occasionally beyond. Although the focus is mainly European, he glances at the Spanish Empire in North America, the Ottomans, and the Chinese. Grygiel offers a fascinating, thoughtful, and incisive analysis—and one backed up with remarkable erudition.

Grygiel understands well how little attention students of politics today tend to pay to premodern states. Recognizing the predominant role that states played in international relations in the 20^th century, scholars see little need to look at the pre-state world. As Grygiel astutely notes, underlying this position is one of two assumptions. The progressive assumption argues that the norms of international relations have grown so gentle in the era of liberalism and the democratic peace that earlier history and its savagery have little relevance. The realist assumption says that human nature is unchanging and is little different today from what it was in Athens or Warring States China, so there is no need to go to the trouble of searching for arcane and ancient examples of interstate behavior when equally enlightening but more familiar cases are at hand. Why study the Peloponnesian War when you can study World War II?

One answer to this last question, we might say, is that the similarity between the two cases is less real than apparent, and that the differences are more instructive. Americans, for example, tend to identify with Athens, the leading democratic state in the Peloponnesian War, but in some ways America in World War II was more like Persia, a neutral power that, by entering into the war late, tipped the balance and decided the outcome at relatively little cost in blood and treasure compared to what other participants paid.

To take a second point, Greek city-states (poleis) were much less unified or centralized than modern states. Civil war, slave revolt, and fifth columns played a central role in the Peloponnesian War but not in the 20^th-century conflict. Greeks in the Peloponnesian War died for honor and glory. Americans in World War II told social-science investigators that they fought for the much more homely motive of helping their buddies in their unit. We could go on and on about the differences between the allegedly unchanging realities of the interstate system in classical antiquity and today and how realists underestimate them, but that is a subject for another day.

Grygiel makes a separate and indispensable point. Like Herodotus, Ibn Khaldun, or Vico, he argues that history is cyclical. In Grygiel’s view, “some strategic realities and actors, which are particular to premodern history,” may be “making a resurgence.” These factors are not much present in the interstate competition of the past several centuries, but they were present in premodern times. In particular, he points to the rise of non-state, alternative sources of allegiance and authority; the possibility of being a player in international affairs without actually being a state; and the accessibility of modern military technology to groups without their own industrial base. And so, we have barbarians.

Grygiel defines barbarians as small highly mobile groups that often were not settled in a fixed place. Perhaps the term that best describes them is warrior bands. They played a continual and important role in premodern history. Scythians, Gauls, Cherusci, Numidians, Huns, Goths, Vandals, Comanches, Saxons, Bulgars, Normans (at least the armed bands that conquered Sicily without a state to support them), and Oirat Mongol tribes are among the peoples whom the author references. Grygiel paints with a broad brush and at times one remembers differences among these groups. Many of them eventually moved from carrying out raids to fighting set battles—and sometimes winning. A prime example is the Goths, who crushed a Roman army and killed the Emperor at the Battle of Adrianople in 378, thereby inflicting on the Romans one of the worst defeats in their long history. Grygiel is aware of such differences, but he rightly focuses on the broader pattern.

In his analysis, barbarians arise in territories not under the effective control of any state. He calls these places ungoverned spaces and argues that they are proliferating today, with the result that new non-state actors are proliferating. He points to destabilization in sub-Saharan and East Africa as well as in Southeastern Europe and Central Asia. As he puts it, failed states are becoming the modern equivalent of barbarian lands in Roman Central Europe or premodern Central Asia, or the North American Plains for the Spanish Empire or the early United States. They offer space for alternate forms of social organization to the state, such as tribes, clans, or religious affiliation. Sometimes they are states within a state, availing themselves of state protection, such as the Taliban in Afghanistan or Pakistan. They use modern technologies, both digital and military. They sometimes have new objectives, whether creating an offshore hegemony (Somali pirates) or a globalized community (al-Qaeda). They are relatively uninterested in diplomacy and, because they are hard to pin down, difficult to deter.

In his study of non-state actors today Grygiel is primarily interested in Islamists and their violent tactics. His use of the term barbarian to describe them is bound to be controversial, but it is defensible.

The term “barbarian” has both advantages and disadvantages. On the one hand, it is offensive, with denigrating connotations of cultural inferiority and brutal behavior. It is also dangerous because it might encourage a state to underestimate the seriousness of the threat posed by supposed inferiors. On the other hand, the term is respectful, because it appreciates the enemy’s military prowess. Barbarians, after all, conquered Rome. The term also captures a state’s ignorance of the nature of the foe. The ancient Greeks invented the term barbarian, which originally referred to peoples whose language they couldn’t comprehend. Grygiel writes: “The term ‘barbarian’ meant that the user of it did not fully understand his strategic interlocutor, the enemy.”

Grygiel concedes differences between ancient and modern barbarians. The latter are more lethal than their predecessors and have a global reach. Nor is there any ancient counterpart to Islamism. But he notes some parallels: localized, individually small, geographically diffuse threats; the preference for raids over invasions; the inclination for terrorizing rather than holding land and administering populations. And of course both groups are violent.

Grygiel sees the barbarian threat in perspective. As he points out, the greatest threat to a state is not the attacks of non-violent state actors but of another state. For the Romans, barbarians on the frontiers were more of an annoyance than a mortal danger until they became organized in quasi-states of their own. In some cases it took centuries for Roman discipline and technique to seep across the frontier before barbarians were able to pose real challenges. By then the empire had suffered enough other challenges to be unable to defend itself. The Romans endured epidemics, currency inflation, political instability, population decline, and de-urbanization. Italy’s population turned away from war to pursue the pleasures of prosperity, and a new military elite from the Balkans replaced it. The new elite tended to prioritize challenges in the east, especially after the creation of a second capital in Constantinople in 330. And they faced a serious challenge indeed: not barbarians but a peer polity.

That challenger arose on Rome’s eastern frontier. The Parthian Empire was an Iranian kingdom that competed with Rome for centuries, sometimes in war, sometimes through negotiations. Then, in the third century, the Parthian dynasty fell and was replaced by a new Iranian kingdom, the Sassanian Empire. The Sassanians were stronger and more effective militarily than their predecessors and represented a major problem for Rome. In fact, the challenge continued past the fall of the Roman Empire in the West in 476. The Byzantines inherited the problem and were continually at war with the Sassanians until the Islamic conquest of the seventh century put an end to the Sassanian Empire.

Grygiel sees an important lesson for today in premodern responses to barbarian attacks: the need to go local. In both ancient and modern cases the threat was a long-term one, “more a condition to manage rather than a problem to solve.” Like ancient barbarians, modern terrorists shrewdly exploit weaknesses in a state’s security system. They often attack local targets and hence force populations to find local solutions. The Romans created frontier troops called limitanei to police the border and respond to raids, and they built networks of border forts. A thousand years later, the Habsburgs used the Uskoks, originally refugees from Ottoman attacks, to defend the border fiercely while acting largely on their own initiative without close supervision from the central government. Grygiel sees such cases as examples of what is needed to stop terrorist attacks today, that is, examples of a principle that he calls security subsidiarity: the work of local police forces, first responders, and, last but not least, an aroused citizenry. The current catch phrase, “If you see something, say something,” comes to mind, but an older saying seems even more apposite: The price of freedom is eternal vigilance.

In Return of the Barbarians Grygiel focuses on violent non-state actors as modern barbarians, but one might extend his analysis to non-violent groups on the move as well: to migrants, for example. After all, some of the ancient barbarians who bedeviled Rome were migrants who sought the security of the Roman Empire from attacks by violent raiders from the steppes. Note that the German term for what English-speakers call the Barbarian Invasions at the end of the Roman Empire is Völkerwanderung, “the migration of peoples,” a rather neutral description. Today migrants to the developed world come, for example, from Central America, sub-Saharan Africa, Syria, and South and Central Asia, and seek entry, legally or illegally, into the security of established states. They want entry to such places as the European Union, Turkey, Australia, Canada, and the United States. Tens of millions of people are on the move these days because of varying admixtures of war, political repression, economic misery, and environmental catastrophe. In future years, perhaps, climate change might lead to even larger numbers of such migrants.

When faced with raiders or migrants, states sometimes build walls. Frontier defenses, from the Roman limes to the Great Wall of China, are examples. These systems can provide a degree of help but never complete security. Often they are just monuments to Ozymandian vanity or strategic exhaustion (think of the Maginot Line). Or consider Athens in 480 B.C.E. Faced with a looming Persian invasion, the Athenians consulted the Oracle of Delphi. The response was to trust in “the wooden walls.” The leadership understood this to mean: build a fleet of wooden ships, and they did. A few diehards holed up on the Acropolis instead behind a wooden palisade. The invaders overwhelmed the palisade and slaughtered the defenders while the new Athenian fleet saved Greece at the Battle of Salamis. The Oracle’s wooden walls weren’t walls at all.

Defense is essential but a defensive mentality is never enough. Instead, prudent states always look outward. They peer beyond the frontier and pay attention to developments in “ungoverned spaces.” Diplomacy, intelligence-gathering, cultural missions, alliances, financial aid, technical assistance, espionage, raids, and sometimes war must always be part of a wise statesman’s toolkit.

Terrorism is part of today’s scene, and a modern Völkerwanderung seems to be in full swing. Both will be with us for the foreseeable future. Let’s hope that we can respond with wisdom and success. First, however, we have to display the humility and the perspective to look back at events from the distant past for some guidance. We have to realize that we are neither as unique nor as progressive as we might think. Grygiel’s insightful analysis, rooted in his profound knowledge of history, offers a precious starting point.