The Second Fall.

Mr. Gress, a senior fellow of the Danish Institute of International Affairs and the Foreign Policy Research Institute, is the author of From Plato to NATO: The Idea of the West and Its Opponents.

The First World War, by John Keegan (Knopf, 512 pp., $35)

The Pity of War: Explaining World War I, by Niall Ferguson (Basic, 520 pp., $30)

The late political philosopher Sidney Hook, though a staunch atheist, referred to the outbreak of World War I as "the second fall of man." The phrasing reflected the profound sense, held by nearly all true democrats who witnessed the effects of that war, that the conflict marked a vast, ominous, and tragic diversion of the course of human history. Before 1914, the liberal principles of free trade, expanding suffrage, and increasing prosperity seemed firmly anchored in not merely the official policies of the great powers, but the very identity and confidence of the peoples of Europe and North America. After 1918, by contrast, Europe sank into stagnation, protectionism, ferocious ideological conflicts, and ultimately a second great war, and the United States withdrew into an isolation that was not broken until Pearl Harbor.

Since the revolutions of 1989 that put an end to Communist power in Europe, historians have begun to speak of the period from 1914-1989 as a "short 20th century," marked by hot or cold war between incompatible ideologies, and above all by a massive assault on liberal democracy, indeed on the entire spirit of liberty, progress, and optimism that characterized the pre-1914 era. Only now, at the end of the 1990s, have international trade and exchange reached the same volume, relative to the GDPs of the great powers, as in 1914. Only now, it appears, is the long deviation finally over.

If, however, the benefits of free trade and self-confident democratic liberalism were as obvious as all that, the great deviation and its trigger, the war of 1914, become almost impossible to explain rationally. They appear either as the violent eruption of repressed barbarism predicted by 19th-century thinkers such as Jacob Burckhardt and Friedrich Nietzsche or, to the religion-minded, as the tragic penalty stemming from the insouciant confidence of those who thought that democracy, prosperity, and free trade were so obviously right that nothing could disturb their onward march.

Although the major questions raised by the war-what caused it, why it lasted so long, how it affected and was affected by culture, economics, and ideology-have been debated since the war itself, clear answers have tended to drown in the clamor of conflicting interpretations. Now, the two brilliant, wholly different, and therefore marvelously complementary volumes by John Keegan and Niall Ferguson present the full record with a comprehensive clarity that will not soon be superseded. These two books will henceforth be the indispensable sources for anyone who wants to begin to understand what the great deviation was and why it occurred.

Keegan deals with the war itself, the strategy and tactics, the fighting, and the morale. Never before has the full panorama of combat in the Great War, from Flanders to Tsingtao and from Jutland to the Falkland Islands, been brought so vividly and passionately to life. This is certainly one of Keegan's best, worthy to stand beside his magnificent debut, The Face of Battle. Few works of history can match the dramatic tension of his account of the German invasion of France in the late summer of 1914. Keegan moves effortlessly from grand strategy to the skills and initiative of individual commanders, such as the German artillery commander Hans von Gronau, who realized on September 5 that the French were about to take the German offensive in the flank, counterattacked against superior numbers, and thus, probably, saved the German army.

Keegan ends his fascinating chronicle with a question. World War I, he says, remains a mystery, in its origins, its course, and above all in the courage and stamina of its combatants. That is where Ferguson picks up the thread. At the end of his account of economics, strategy, war finance, patriotism, and morale, the war remains a challenge and above all a tragedy, but it is, at least, no longer completely mysterious.

Ferguson, an economic historian whose grandfathers fought in the war, revises a number of received opinions-for example, the notion that Germany was starved into defeat by the British blockade, or that the peace of Versailles was vindictive and therefore to be blamed for German revanchism and the rise of Hitler. Germany in the 1920s was strategically and economically better off than in 1914, largely thanks to the collapse of Russia, which had not yet reemerged as the Communist superpower. It was the German hyperinflation of 1920-23, not the Versailles Treaty, that paved the way for Hitler, and, as Ferguson has argued convincingly in other works, hyperinflation was in no way a necessary consequence of the war.

Before 1914, liberal thinkers such as Norman Angell argued that war between industrial powers made no sense and that the only rational way to seek power was by trade and economic growth. Angell was right, but he underestimated the power of fear, nationalism, and the alliance commitments that drove Germany to support Austria-Hungary against Serbia and Russia to threaten Germany in response. "The banks could not stop a war," writes Ferguson, "but war could stop the banks." Economic interests trump political passions only when economics has itself become the political passion. That was not the case in 1914; whether it is the case now, time will tell.

Anyway, once the war got started the economic-interest calculus changed. As a journalist wrote in 1917, "Since nations counted money no more than pebbles on a beach, and all would probably repudiate in one form or another at the end of the war, there seemed no reason for stopping, especially as so many people were growing rich by the war; the ladies liked being without their husbands, and all dreaded the settlement afterwards." Ferguson endorses this judgment and its corollary, that the only way to end the war, once it started, was by defeating the main forces of the enemy. And that is what happened.

Victory came at a tremendous cost. One of Ferguson's more interesting innovations is to calculate the cost per death inflicted for the various belligerent states. He shows that the Central Powers killed more cheaply than the Entente. (The figures are $36,485 for every dead German or ally versus only $11,345 for the Central Powers to kill a soldier of the Entente.) Therefore, though the Central Powers were outnumbered, it took a long time to defeat them.

Ferguson's volume is a genuine mine of information, joining informative charts and tables to fluent analysis. But it is not problem-free. Ferguson is close to the John Charmley school of younger British historians who see the 20th century as a long and avoidable chronicle of British imperial decline. If only Britain had stayed out of the European wars of 1914 and 1939, this school argues, Britain would not have suffered a terminal loss of world power. A German victory in World War I, likely if Britain had stayed out, would not have been so bad: It would merely have produced, 80 years earlier, the German-dominated European Union of today. There would have been no Hitler, no second war, no Soviet power in Central Europe, a lot more prosperity and, yes, democracy, and a lot more people living their natural span.

This kind of counter-factual history, a speciality of Ferguson's, is often challenging and helpful; the Charmleyites may well be right that a more multipolar world, in which Britain still held a global position and American hegemony were balanced by others, might be preferable to what we have. But to equate the militant and proud imperial Germany of 1914, stretching from Strasbourg to Konigsberg, to the flaccid and effete rump Germany of 1999 is to ignore what is decisive in history. It is not clear to this reader, at least, that German hegemony in 1914 would really have been as benign as the waning German economic hegemony of today.