Civil Wars: A History in Ideas. By David Armitage. Knopf, 349 pages; $27.95. Yale University Press; £18.99.

IN DECEMBER 2011, months after fighting broke out in Syria, a State Department spokesman was asked if the conflict was really a civil war. He dodged the term, which is fraught with legal, military, political and economic implications for the intervention of outside states. Bashar al-Assad called his enemies “terrorists”. The Syrian people understood their conflict more hopefully, as a revolution (though one exile insisted to the Guardian: “This is not a revolution against a regime any more, this is a civil war.”) In July 2012, after 17,000 deaths, the Red Cross at last acknowledged that Syria was engaged in “armed conflict not of an international character”.

Civil war, writes David Armitage, a historian at Harvard University, is “an essentially contested concept about the essential elements of contestation”. Intrastate war has replaced wars between states as the most common form of organised violence: the annual average of intrastate wars between 1816 and 1989 was a tenth of the number in each year since 1989. Only 5% of wars in the recent period have been between states. But an abundance of cases has not improved clarity. “Civil war” can be invoked to bring a conflict within the constraints of the Geneva Convention and to authorise intervention, including millions of dollars in humanitarian aid. But it can also be used to dismiss conflicts as internal matters, as happened with Rwanda and Bosnia in the 1990s. It is rewritten as “revolution” when rebels are victorious—but was the American Revolution not a civil war within the British empire? Ruling powers, quick to deny the legitimacy of their challengers, reduce it to illegal insurrection. “Civil war”, by contrast, recognises rebels as an equal, opposing party—in effect, a separate nation.

In “Civil Wars” Mr Armitage traces the evolution of an explosive concept, not to pin down a proper meaning but to show why it remains so slippery. The Romans, to whom he attributes the origin of the idea, spoke of bellum civile with horror: a conflict against enemies who were really brothers, for a cause that, consequently, could not be just, it defied their very criteria for war. It was the savage, suicidal turning of a civilisation on itself. Yet, it seemed an inescapable feature of Roman civilisation; its foundational curse, a recurrent phenomenon like the eruptions of a volcano. “No foreign sword has ever penetrated so,” wrote the poet Lucan. “It is wounds inflicted by the hand of fellow-citizen that have sunk deep.” Their corpus of pained reflections meant civil war was long viewed through “Rome-tinted spectacles”.

The age of revolutions in the late 18th century recast civil war as part of a visionary programme of change and emancipation. But the forward-looking idealism of the Enlightenment did not banish the senseless barbarism of civil war so much as create new conditions for violence. It is hard to disregard the sense that revolution, for all its Utopian promise, is merely a species of civil war.

International law has attempted to civilise civil war. But as Mr Armitage reminds readers, the modern order rests on sovereign inviolability and the pursuit of human rights, two principles that are in conflict, making clear guidelines elusive and incomplete. The original Geneva Convention of 1864 did not even extend to civil wars: “It goes without saying international laws are not applicable to them,” explained a drafter. Today’s legal protocols may only make leaders avoid the term, complicating the humanitarian response it is meant to trigger.

Globalisation has added further conceptual twists. The first world war, John Maynard Keynes said, was really a “European civil war”. In the view of Carl Schmitt, a German political theorist, Leninist socialism unleashed a “global civil war”. To many today, transnational terrorism is another kind of civil war without borders. Foreign intervention means that even conflicts that begin within borders increasingly spill beyond them, with reverberations across the globe. In 2015, 20 of 50 internal conflicts were internationalised civil wars. The Roman notion of civil war as a wound that never quite heals haunts these conflicts and politics itself, which is, in the words of Michel Foucault, just civil war “by other means”. In an era of transnational populism and anti-globalist revolt, this notion is resonant. The meaning of civil war, as Mr Armitage shows, is as messy and multifaceted as the conflict it describes. His book offers an illuminating guide through the 2,000-year muddle and does a good job of filling a conspicuous void in the literature of conflict.