For historians, debating the causes of wars is a kind of endless intramural sport. Less common are debates over the causes of peace. Now James J. Sheehan, a professor of history at Stanford and former president of the American Historical Association, has written a new thematic history of twentieth-century Europe that is framed around the question of peace. He tries to explain why Europe, a continent long plagued by war, has been at peace since the middle of the twentieth century. This “eclipse of violence,” writes Sheehan, has been “a slow, silent revolution, hidden in plain sight, but...nonetheless a revolution as dramatic as any other in European history.”

It’s difficult to look back through the first half of the twentieth century, with its catalog of genocide and war, and recall that the century began with some enthusiasm for peace. From the end of the Franco-Prussian war in 1871 until the outbreak of World War I in 1914, Europe had enjoyed a long, unprecedented period of peace. Many people believed that global commerce and historical progress had made war unnecessary and avoidable. Pacifism—the term came into use in 1901—gained adherents worldwide.

All of that was shattered by the outbreak of the Great War. The habit and expectation of war among most European leaders counted for more, finally, than the growing number of Europeans inclined toward peace. Most European soldiers marched off to the trenches with enthusiasm. Sheehan recounts the horrors of this war in gruesome detail, and argues that European disillusionment with war began with World War I. The lesson of that war, according to both John Maynard Keynes and Norman Angell, was that in modern war no one wins; even the victors’ fruit is bitter. As they recovered from the shock of World War I, many Europeans could not believe that they would become involved in yet another war.

But they did—and this time on an even greater scale, though in World War II the armies went marching off with much less enthusiasm. Between the two world wars, advocates for peace had gained some ground. Fear of another war was so great that British Prime Minister Neville Chamberlain was considered a hero for compromising with Germany in order to secure peace for Britain. Sheehan quotes French novelist Roger Martin du Gard, who wrote to a friend, “Anything rather than war!..even fascism in Spain.... Anything, Hitler rather than war.”

Still, in the years before World War II, violence was again endemic. The civil war in Spain, the purges in Russia, and the Italian invasion of Ethiopia were all vicious but localized conflicts that were contained because Europeans did not want another general war. Finally, in 1939, the struggle between pacifism and militarism was over: Europe went to war. With deliberate ambiguity, Sheehan calls World War II “the last European war,” which is also the title of his chapter on this conflict. According to Sheehan, the collapse of the French army in 1940 was as much a defeat of the will and spirit as a failure of arms and tactics. The leaders of the French military were, in the words of the French historian Mark Block, “incapable of thinking of a new war.”

Sheehan piles the horrors of World War II on top of those of the First World War and then describes a postwar Europe where a real consensus against war solidifies. Sure, war and violence continue to flare up around the edges of Europe—in the Balkans, in Northern Ireland, and in European colonies in Africa and Asia. But after World War II there is a change of mindset. Algeria is not worth the cost in lives and treasure, says de Gaulle. The British colonies are soon cut adrift. Vast forces of tanks and troops are deployed in Germany during the cold war, yet war is nearly unthinkable. Clashes in the Balkans in the 1990s do not spread into other parts of Europe. In the 1980s and ’90s, governments in Europe began cutting their defense expenditures, often drastically. The primary role of the state was no longer to field an army but to provide for health care, housing, and a vibrant economy. Thus, the nations of Europe became “civilian states,” to use Sheehan’s term, and the status of the military faded. Even the vividly colored uniforms of the military, which had been so visible in the elite social life of European countries before World War II, started to disappear (except among the British). Today, the drab uniforms of most European soldiers could be mistaken for those of postal workers.

It’s a commonplace to credit the formation of the European Union with the preservation of peace. The elimination of physical borders, the free passage of goods and people, the (almost) common currency, the new EU political structures—all prevent European nations from reverting to war with each other. But Sheehan argues that this thinking is backwards: the emergence of a new Europe “was not the cause of the long peace after 1945; peace was the new Europe’s necessary condition.” Unity did not produce peace; rather, peace allowed Europe to unite.

For Sheehan, the important thing about the European Union is that it has declined to develop into a military and political power on the old model of the nation-state. All the talk of creating a rapid-response force for the EU, independent of NATO and the Americans, hasn’t yet translated into many real soldiers. When voters in France and Denmark voted down a new EU constitution that might have transformed the EU into a unified superpower, they were voting against a larger military, among other things, and in favor of democratic states devoted, above all, to protecting the civilian interests of their citizens.

Sheehan disagrees with Raymond Aron, who wrote in the 1950s that “the European idea is empty...it has neither the transcendence of messianic ideologies nor the immanence of concrete patriotism.” The idea of Europe, writes Sheehan, does not “stir people’s hearts as nations sometimes have done. It is not something for which many would have been willing to give their lives.” Still, the European idea is not empty. Its substance is not “national enthusiasm and patriotic passion” but rather “widespread commitment to escape the destructive antagonisms of the past and a deep concern for those economic interests and personal aspirations that dominate public life.”

This very smart and clearly written book reveals a Europe so fundamentally transformed that future historians may never need to debate the causes of a twenty-first-century European war.

Related: James J. Sheehan reviews Michael Burleigh's Earthly Powers