“Four score and seven years ago,” Lincoln said at Gettysburg on November 19, 1863, “our fathers brought forth on this continent, a new nation, conceived in Liberty, and dedicated to the proposition that all men are created equal. Now we are engaged in a great civil war, testing whether that nation, or any nation so conceived and so dedicated, can long endure.” The power and beauty of Lincoln’s speech have made those words among the most memorized and analyzed in American history. Yet as David Armitage points out in his lucid and learned book Civil Wars: A History in Ideas, we often gloss over a crucial phrase: “great civil war.” More than 150 years later, with the Civil War’s outcome undisputed, it is easy for us to take that term for granted. But at the time and for many decades after the conflict, it was referred to by a variety of names that each carried heavy interpretive loads: rebellion, revolution, war.

CIVIL WARS: A HISTORY IN IDEAS by David ArmitageKnopf, 368 pp., $27.95

From the Union side, it could be seen as a rebellion by the South, subject to criminal prosecution under domestic law. Lincoln himself used “rebellion” far more often than “civil war,” and the official War Department history compiled in the late nineteenth century was called The War of the Rebellion. Meanwhile, from the Confederate side, it could be framed as a standard international war between different peoples, as the name “War Between the States” implied. As Armitage shows, describing the conflict as a “civil war” had important foreign and domestic implications, simultaneously affirming that both sides “remained members of the same political community” and highlighting the unity of the nation. The Union’s victory secured those meanings for the war, and they were further ratified when Congress finally settled on the official name of “Civil War” in 1907.

The questions the Civil War raised remain familiar to us today because they come up inevitably whenever we see internal conflicts within a country. Is the country one community or several? Is the conflict subject to domestic criminal law or the international laws of war? Could other countries legitimately intervene, and if so, using what justification? Such internal conflicts are becoming more and more common. By one measure, nearly 95 percent of all conflicts since the end of the Cold War have taken place within states rather than between them. As in places like Somalia, Rwanda, the Balkans, and now Syria, these wars often spill over their borders, either by drawing in other countries or sending out millions of refugees.

To cope with the seemingly intractable problem of civil war, lawyers have tried to fit it into the framework of international law, and social scientists have tried to define it precisely using data. Yet the ongoing confusion over Syria and other conflicts around the globe have eluded these attempts to impose rationality on ragged chaos. Armitage proposes a different approach: Can a history of ideas about civil war give us any guidance?

The term civil war originates in ancient Rome. The first recorded use—the Latin bellum civile—came in 66 BCE, when Cicero deployed it in a speech in the Forum. The way he used the term, without defining or explaining it, suggests the concept was familiar to his audience. Rome had experienced plenty of internal conflict between patricians and plebeians since its founding, but none rose to the level of war until 88 BCE. That year, the Roman general and consul Sulla opposed legislation introduced by the tribune Sulpicius, who wanted to extend the franchise. A riot broke out and Sulla withdrew from the city with his troops. He soon marched his army back to the city and took the Capitol. He declared his opponents public enemies and used dictatorial powers to undo Sulpicius’s laws. Then he sent his troops away and restored Rome’s usual governing structure.

As civil wars go, this first one was relatively short and bloodless. But it marked an important turning point in Roman history, and its effects would be felt for centuries to come. Sulla’s actions helped define the concept of civil war. For the first time, an army of Romans had marched on their fellow Romans in a battle for control of the Republic. “Trumpets and standards were the visible signs, conventional warfare the means, and political control of the commonwealth was the end,” Armitage writes. “All told, these were the peculiar marks of civil war as opposed to mere tumult, dissension, or sedition.”

Sulla’s actions later seemed to mark the start of a cycle of civil wars from which Rome could not escape. The two consuls whom Sulla left in command of Rome clashed, and the Senate declared one of them a public enemy. That consul marched an army to the city to restore himself to power. Then Sulla was declared a public enemy, eventually prompting him to march on Rome again and install himself as dictator. All this was just in the 80s. In 49 BCE, Julius Caesar followed Sulla’s example when he crossed the Rubicon to take his army into Italy, sparking a twenty-year period of almost constant civil wars that led to the end of the Republic. Regular civil wars would then rack the Empire throughout its existence.

For centuries, Roman writers struggled to discern the meaning of civil war. Some adhered to what Armitage calls the “republican” view: Maybe to be civilized at all meant to be susceptible to cycles of civil war. Others pushed an “imperial” narrative: Civil wars afflicted republics but could be cured by a strong monarch or emperor. Finally, in the 400s CE, Augustine developed a Christian interpretation of civil war as an inevitable feature of any city dedicated to this world instead of to God. As later generations of Europeans tried to make sense of their own civil wars, they inherited these visions of “history structured around an ethically challenging, apparently recurrent phenomenon that was, nonetheless, the paradoxical mark of civility.” Whatever the cause, civilization and civil war seemed intimately connected.

The modern international order depends on two principles that are often in tension. Civil war brings them into open conflict. The first of these is sovereign independence, or the right of a state to exercise power within its own jurisdiction free from outside interference. The second is respect for human rights, which entails that the international community can intervene to stop clear violations.

These principles can be traced back at least as far as the largely forgotten eighteenth-century Swiss writer Emer de Vattel, who emphasized that nations should be “free and independent.” Benjamin Franklin sent a copy of Vattel’s 1758 treatise The Law of Nations to the Continental Congress in 1775. A year later Vattel’s work influenced the language and purpose of the Declaration of Independence, which used an argument about “unalienable Rights” to assert that the United States were now “Free and Independent.”

Vattel’s main goal in his discussion of civil wars was to bring them under international law by reframing them as standard international conflicts. This was what the Declaration of Independence accomplished in fact: It said that a civil war within the British Empire was actually an international war between separate countries. Vattel said that a mere “sedition” or “insurrection” escalated to a civil war when the rebels fighting against the sovereign had justice on their side, usually as the result of some train of evils or abuses. For Vattel, there was essentially no distinction between a civil war and an international war, since a civil war already represented the splintering of a single political community into two separate and hostile societies. International law rather than domestic law should govern the conflict, and foreign states should feel free to assist one side or another just as they would in a standard war.

The implications of Vattel’s argument were huge. By his logic, foreign powers could intervene at their own discretion in the internal conflicts of other countries. All they had to do was declare that the conflict rose to the level of a civil war and that one party had right on its side. In 1778, France became the first country to recognize the Declaration of Independence and entered an alliance with the United States, helping ensure that the war would be remembered as a successful revolution and not simply a failed civil war.

More than 80 years later, Vattel would have declared that the American Civil War was also a case of two separate, warring nations in which international law, not domestic law, should apply. The Union general and international lawyer Henry Halleck disagreed, contending that the right of foreign powers to recognize both parties in a civil war as independent states was a recipe for chaos. The question of whether the conflict was a rebellion or a civil war had immediate practical implications: Should rebels be accorded the rights of lawful combatants, or should they be prosecuted as criminals?

In 1862, Halleck asked the German American political scientist Francis Lieber to draw up a military code to guide the conduct of Union troops. The result, as John Fabian Witt details in Lincoln’s Code (2012), became the foundation for the modern laws of war. “Civil war is war between two or more portions of a country or state,” Lieber wrote, “each contending for the mastery of the whole, and each claiming to be the legitimate government.” The precise details in this definition justified the Union’s stated position that the South was in rebellion: The conflict didn’t count as a civil war because the South wasn’t “contending for the mastery of the whole.” But all around that passage, Lieber located civil war on a slippery sliding scale in which an insurrection could grow into a rebellion and a rebellion could develop into a war. That confusion better reflected the inconsistencies that inevitably cropped up in both terminology and policy on the Union side.

The American Civil War formed part of a global outbreak of conflict in the mid-nineteenth century, which spurred international efforts to make conflict more humane. In 1863, the Red Cross was founded, and the First Geneva Convention was held in 1864. After World War II, similar international organizations and agreements took on greater importance at the same time as nationalism, decolonization, and Cold War rivalries caused a spike in “non-international armed conflict.” Armitage describes various groups’ attempts to establish principles and protocols governing intervention in civil wars, and he recounts efforts to provide a precise definition of civil war based on counts of casualties, boundaries, and participants. By one metric, there have been only three civil wars in modern history, while another argues there have been seven in the last half-century of Iraqi history alone.

The lingering question is whether any of this history can help us when we’re trying to decide what to do about conflicts like the civil war in Syria. Armitage helps us understand the different ways civil war has been defined and why the label “civil war” now carries so much weight. Thousands of lives hang in the balance whenever the Red Cross decides whether or not a given conflict is a “civil war” to which the Geneva Conventions apply. The concept of civil war, Armitage argues, has become the lens through which we view conflicts and the “weapon with which the rhetorical battles over their significance [is] waged.” This seems right: the modern political order is built, after all, on the deep desire, identified by Hobbes in 1651, to avoid civil war at all costs. Within that order, as Armitage shows, civil war has also played a key role in defining the modern concept of revolution and drawing the boundaries of political community in a globalized world.

Over the course of the eighteenth century, the optimism of Enlightenment social and political thought gave birth to a new idea of revolution that stood in stark contrast to the seeming senselessness of civil war. Although violent revolution and civil war amount to essentially the same thing in practice—internal conflicts over control of the political community—they became distinguished in thought by the bald assertion that revolution was constructive, hopeful, and progressive while civil war was destructive and divisive. In this view, revolution involved conscious choice rather than passive suffering; it could be directed rather than endured. Revolutions fit into a story not of “strife but of modern emancipation.” It has become far easier to excuse violence perpetrated in the name of self-consciously progressive revolutions, even though they are really no different from civil wars—except for which side wins.

If civil war has any constructive connotation, it is that describing a conflict as a civil war paradoxically emphasizes the essential unity of the combatants. Especially when applied to foreign wars in a metaphorical sense, the phrase highlights that the people tearing each other apart share a common culture and political community. In 1919, for example, John Maynard Keynes referred to World War I as “the European Civil War.” Later, Dean Acheson referred to the whole period from 1914 to 1945 as a long “European Civil War” that became bound up with a separate “Asian Civil War.” This idea expanded to the global scale during the Cold War, which was often depicted as a worldwide struggle between capitalism and communism, and today some political writers use “global civil war” to describe the ongoing conflict between established states and transnational terrorists.

The idea of global civil war isn’t “susceptible to analytical measurement,” Armitage writes, nor is it “subject to legal regulation or humanitarian amelioration.” But it does carry political meaning, perhaps even a moral imperative. A character in Victor Hugo’s Les Misérables wonders, “Is not every war between men, war between brothers?” At the root of these expansions of the meaning of civil war lies the powerful idea of a single, universal human community.