In 1816, as Europe emerged from 20 years of war and revolution, the French political theorist Benjamin Constant wrote an essay comparing ancient and modern ideas of liberty. The modern European differed most fundamentally from the Athenian and the Roman, Constant argued, in his attitude toward violence. In the ancient world, being at war was a normal state: “Each people incessantly attacked their neighbors or was attacked by them … All had to buy their security, their independence, their whole existence at the price of war. This was the constant interest, the almost habitual occupation of the free states of antiquity.” As a result, the right that mattered most to a Greek or Roman was the right to participate in the government of the state, including when and how it waged war.

The modern European, on the other hand, Constant wrote, saw war as a nuisance and an aberration, and looked forward to a day when it would be completely abolished. An advanced, 19th-century people “is strong enough to have nothing to fear from barbarian hordes. It is sufficiently civilized to find war a burden. Its uniform tendency is towards peace.” Even the divisions between nations and states, Constant believed, were beginning to look anachronistic: “While each people, in the past, formed an isolated family, the born enemy of other families, a mass of human beings now exists, that under different names and under different forms of social organization are essentially homogeneous in their nature.” Even though he was writing just a year after the end of the Napoleonic Wars, Constant was confident that Europe’s future lay in a union of pacific, commercial nations.

Constant’s diagnosis was 150 years premature, but it was not wrong. Though his name does not appear in “Where Have All the Soldiers Gone?” (Houghton Mifflin, 284 pages, $26), James Sheehan’s important new book can be read as a vindication of Constant’s prophecy. For Mr. Sheehan tells the story of how, after two world wars, Europe realized that its “uniform tendency” was toward peace, embraced the “homogeneity” of its different peoples, and finally decided that modern warfare was “a burden” too great to be borne. The 20th century was the most violent in European history, yet as we enter the 21st, the continent is more peaceful, prosperous, and free than it has ever been. For the first time, we can be confident that, if the world plunges once again into the abyss of war, it will not be because of the rivalries of the European powers.

In his brief and lucid narrative, Mr. Sheehan, a professor of German history at Stanford University, takes the reader through the stages of this transformation. The story he has to tell is hardly an unknown one. In fact, for most of its length, “Where Have All the Soldiers Gone?” is essentially a history of the two world wars and the Cold War. For it was these three wars — the first two fought by Europeans, the second by America and Russia using Europe as a virtual battlefield — that finally cured the continent of its ancient addiction to violence. It is useful to remember, in these days when Europeans are so frequently to be heard attacking American militarism, that Europe did not so much renounce its military power as immolate it.

Indeed, if you had told the average Frenchman or German in 1908 that, in 100 years, his country would have practically disarmed itself, he would never have believed it. Not that he would necessarily have deplored the prospect. As Mr. Sheehan writes in his book’s opening chapters, the turn of the 20th century saw a flourishing peace movement in Europe, inspired by the Constant-like belief that the continent was too civilized, and too commercially interdependent, ever to go to war again. Now-obscure writers such as Ivan Bloch, author of the 4,000-page tract “The Future of War,” and Norman Angell, whose “The Great Illusion” was a best seller in 12 languages, held that modern warfare had become too costly to make sense as a solution to Europe’s problems. Any war involving breech-loading rifles, heavy artillery, and other high-tech weapons, Bloch predicted, would lead to “not the slaying of men, but the bankruptcy of nations and the breakup of the whole social organization.”

This prophecy was entirely correct, but it did not stop the governments of Europe from marching in lockstep into World War I. The problem was that pacifism remained much less influential in Europe than militarism — a public culture that glorified the military virtues, and held that the first purpose of the state was to make and win wars. The face of every European capital before 1914 was dominated by war memorials and army parades. More important, every male citizen of most European countries spent years doing military service, learning the lessons of what the president of France called, in 1894, “this grand school of patriotism, the army.” Those lessons were so well learned that, when war broke out in August 1914, the public of every belligerent country responded with delirious enthusiasm. The cheering crowds in Berlin were matched by those in Saint Petersburg; Rupert Brooke’s patriotic raptures echoed those of Sigmund Freud, who wrote, “All my libido is given to Austria-Hungary.”

The war that ensued, however, was not the war those crowds had bargained for. It was, rather, what H.G. Wells called “Bloch’s war,” an interminable, ruinous stalemate. Nine-and-a-half million men perished in 1914–18, along with the governments of Russia, Austria, and Germany, the self-confidence of Britain and France, and the assumption that it was Europe’s right to rule Africa and Asia. As Mr. Sheehan writes, none of the powers that started World War I, not even the victors, would have done so if they could see how the war was going to end.

It is no wonder, then, that, after World War I, most Europeans revolted against the old culture of militarism. Instead of triumphal arches, the distinctive monuments to the Great War were tombs to the unknown soldier, a sign of the shift in European attitudes toward the casualties of combat. “Until the twentieth century,” Mr. Sheehan points out, “the nameless corpse left on the battlefield was the accepted norm, not the symbolically powerful exception.” The survivors wanted nothing more than to avoid another conflict, at practically any cost. The prime minister of Britain, Stanley Baldwin, predicted in 1926 that “one more war in the west and the civilization of the ages will fall with as great a shock as that of Rome.” In 1928, in the Kellogg-Briand pact, the nations of Europe promised to renounce war forever.

Mr. Sheehan makes a convincing case, then, that the real origins of contemporary European pacifism lie as far back as the 1920s. The problem was that while Britain and France were tired of their victories, Germany had never been reconciled to its defeat. Hitler, a veteran of World War I who had been decorated for heroism, came to power on the understanding that he would re-fight that war, this time to its predestined, victorious conclusion. It is a measure of how deeply unmilitary the Western democracies had become that they proved totally unable to meet Hitler’s challenge, letting him get away with aggression after aggression before they finally fought back. When war did come, France, which had defeated Germany at such tremendous cost 20 years earlier, collapsed in a matter of weeks, unable to bear the strain of a second effort.

Not until World War II had been won, thanks mainly to Soviet lives and American wealth, was Europe able to resume its course toward permanent peace. The defeat and partition of Germany finally put an end to her expansionist ambitions: For the first time since Napoleon, France and Germany agreed to stop being enemies, choosing instead to work together in building the European Economic Community and the European Union. This resigned neutrality allowed Europe to direct ever more of its wealth away from defense spending and into social welfare: From 1955 to 1979, Mr. Sheehan notes, the military’s share of Britain’s budget fell from 25 percent to 10 percent.

The rise of what Mr. Sheehan calls “the civilian state” went along with a transformation of public attitudes. Before the Great War, the source of a state’s legitimacy lay in its ability to make war: “Without war,” as the German historian Heinrich von Treitschke put it, “there would be no state.” After World War II, on the other hand, Mr. Sheehan writes, “the legitimacy of every western European government depended on its capacity to sustain growth and prosperity.” Europeans stopped caring about the military virtues, and being a soldier became just another job; in the Netherlands, soldiers even have a labor union.

All this is, of course, cause for great celebration. Hundreds of millions of people in Europe are now safer, healthier, and happier than their ancestors ever were. Billions more in Asia and Africa are free from the yoke of European imperialism (though their new rulers might not be much better). Yet, as Mr. Sheehan points out, this epochal endorsement of peace was sponsored by a different kind of war — the Cold War, which turned America into the guarantor of Western European freedom. Without NATO and the American nuclear umbrella, Europe’s transformation would never have been possible. The continent could afford to discard the military virtues, not because they were no longer necessary, but because they were outsourced to America, which for 50 years managed to combine strength with civility more successfully than Europe itself ever did.

The question that arises at the end of “Where Have All the Soldiers Gone?,” and that motivated its writing in the first place, is whether Europe’s pacifism is still a viable course now that the Cold War is over. Fortunately, it does not seem that Europe’s federal institutions are in danger of coming unglued. As Mr. Sheehan writes, “the institutional integration of postwar Europe not only survived the end of the Cold War but became stronger and more extensive.” A new war of aggression by Germany is still unimaginable.

But the European disavowal of force has also left it impotent in the face of subtler challenges. As Mr. Sheehan details in his closing chapters, modern Europe, which was born morally and politically in reaction to the Nazi genocide, proved itself simply unwilling to stop the genocide in Yugoslavia. A terribly emblematic moment came in 1995, when Dutch peacekeepers in Srebrenica stood by while Serbian troops slaughtered thousands of Muslims. Only American intervention put an end to the Bosnian war, just as only American power and initiative drove Saddam Hussein from Kuwait, stopped Milosevic’s depredations in Kosovo, and expelled Al Qaeda from Afghanistan.

Especially since the attacks of September 11, 2001, Europeans have retreated into self-righteous inactivity, happier to condemn American actions than offer a constructive alternative. The threat of Islamic terrorism, of a nuclear Iran, of resurgent authoritarianism in Russia — all have been met with equivocation by most European governments. (The exception was Tony Blair, whose fall demonstrates how little tolerance even Britain now has for the projection of military power.) As Mr. Sheehan shows, there is no chance that this will change anytime soon. After endless conferences and resolutions, the European Union has been unable to organize an armed force the size of an American Marine brigade. “The European Union may become a superstate,” Mr. Sheehan concludes, “but not a superpower.”

In this way, too, Europe seems to be fulfilling Benjamin Constant’s prophecy. Constant foresaw that “the danger of modern liberty is that, absorbed in the enjoyment of our private independence, and in the pursuit of our particular interests, we should surrender our right to share in political power too easily.” If contemporary Europe is in some sense more modern than America, as its partisans take pride in boasting, it is modern also in the ambiguous sense that Constant had in mind 200 years ago: “Individual independence is the first need of the moderns. Consequently one must never require from them any sacrifices to establish political liberty.”

akirsch@nysun.com