In 1836, Alfred de Musset created two fictional blockheads, Dupuis and Cotonet, and allowed them to make themselves ridiculous in the pages of the Revue des Deux Mondes, trying to define “Romanticism.” At first, they “thought that romanticism meant imitating the Germans.” Then, in 1830–31, they were sure it meant writing historical novels about “Charlemagne, Francis I, or Henry IV.” Then it occurred to them that “romanticism might be a system of philosophy and political economy.” But on further reflection, it seemed more likely to have “consisted in not shaving, and in wearing waistcoats with long, stiffly starched lapels.” In despair, they finally wondered, “Is it anything, or is it only a fine-sounding word?”

Defining Romanticism has not gotten any easier since 1836, but neither has our sense of its importance diminished. Isaiah Berlin thought Romanticism was “the greatest single shift in the consciousness of the West”; Kenneth Clark believed that it introduced an entirely new sensibility into European art. But what it certainly was, at the very least, was a revolt against the Enlightenment — against the bourgeois capitalism the Enlightenment had turned into the stuff of heroism, against natural law and natural rights, and against the balance and predictability that Newtonian science had imparted to the 18th-century world. It clothed itself, as so many revolts do, in the costume of what it deemed an unjustly despised past — Hugo’s medieval Paris, Ossian’s epics, the Grimm brothers’ German fairy tales — but its real grasp was for the future, a future that would glorify the politics of race and blood, the philosophy of Dionysian passion, and the aesthetic of the mysterious.

The bulk of Tim Blanning’s long career as a historian at Cambridge has been devoted to the closing scenes of the ancient regime — to Joseph II of Austria (the Enlightenment’s model emperor), to the onset of the French Revolution and its wars, and to the reorganization of the German cities and states after Napoleon’s dismemberment of the Holy Roman Empire. But he has also had a longer view for the history of European culture (as the editor of the Oxford History of Modern Europe), and especially for 19th-century music. All of this puts him in a particularly good position to speak of the arrival of Romanticism on the European scene, and he does so with a verve, a breadth, and an authority that exceed every expectation of what might otherwise have been an indecently brief slide show of the Romantic revolution.

Picking a starting point for Romanticism has long been a favorite parlor game: For Berlin, it was Herder and Kant; for Clark, it was alternately the Lisbon earthquake, the nightmare in 1764 that set Horace Walpole to writing The Castle of Otranto, Burke’s Inquiry into the Origins of the Sublime (1757), and Piranesi’s Imaginary Prisons (1749). For Blanning, it was the day in July 1749 that the eye of Jean-Jacques Rousseau was caught by an advertisement for an essay contest on the question: “Has the progress of the sciences and arts done more to corrupt morals or improve them?” The promoters of the contest were confidently expecting variations on improve; it suddenly swam into Rousseau’s head that the real answer was corrupt, and from there flowed a lifelong declaration of war against reason, calculation, balance, law, and orderliness, which Rousseau believed had snatched away the “innocence” of humanity.

Isaiah Berlin was deeply suspicious of fingering Rousseau as Romanticism’s progenitor, and Rousseau merits only one passing reference in Clark’s The Romantic Rebellion (1973). And Blanning himself introduces a fairly extensive supporting cast for Romanticism’s debut, including Wordsworth (on the sublime), Hamann (on passion), Johann Heinrich Merck (on the deadness of reason), and Kant. But his fundamental instinct for fixing on Rousseau is, I think, straight and true, for Rousseau represented a repudiation of everything the Enlightenment held at its core. Chief among those antagonisms was Rousseau’s (and Romanticism’s) hostility to both democracy and commerce. In a world of natural plenty, Rousseau believed commerce created artificial scarcity (Locke had believed the exact opposite — that this was a world of scarcity that commerce and property turned into a cornucopia). Those who led commercial lives did so under the most deadeningly and harshly rational rule of all, the bottom line, which reduced Nature to mere utility.

The bourgeoisie, therefore, was nothing more than what Goethe called “the gawping public.” What Rousseau hoped to be ruled by was a mysterious “general will” emerging from “the people” as a tribal mass, not by the checks and balances of individuals and their representatives. Nations, as culturally defined organisms rather than assemblies of free and equal citizens, became the object of Romantic ardor.

One of the most remarkable aspects of Blanning’s little book, and the one most often missing from historical surveys of Romanticism, is his easy grasp of Romantic music, and particularly Beethoven, Wagner, and Berlioz. Only Jacques Barzun (and, in a more strictly academic sense, Charles Rosen) are more effective at connecting the dots between the likes of Goya and Friedrich on one hand, and Carl Maria von Weber and E. T. A. Hoffmann on the other. If there is a serious gap anywhere, it lies in Blanning’s failure to draw Romantic politics into this net. For certainly, authoritarian notions of society and polities built onBlut und Eisen feed their souls on a Romantic rejection of democratic universalism and natural law. In that sense, Romanticism’s darkest legacy is the one that stained the 20th century with fascism and socialism.

The Routledge Encyclopedia of the Romantic Era takes 1850 as a cutoff date for the Romantic revolution, and there was at that time no shortage of voices that wept for Romanticism’s demise at the hands of the triumphant bourgeoisie. Blanning is unconvinced: Rousseau’s curdled contempt for reason may have lost its initial momentum a century after his epiphany on the road to Vincennes, but not its enduring attraction, something that Blanning picks out unerringly in Schoenberg, Gropius, and postmodernism. “All postmodernists have in common a rejection of grand narrative, teleology, and rationalism,” and when postmodernism is mixed with “a vaguely leftist” touch of “anarchic hedonism,” the result belongs “squarely . . . in a line that stretches back to . . . romanticism.” I should add only this: that the sound we hear, pulsing beneath, is the thump of jackboots.