However, once the howitzers had started their bellowing, proletarian internationalism had a marked tendency to evaporate. Only Lenin and a handful of other irreducible revolutionaries bided their time, waiting for the war to devour those monarchs who had been foolish enough to start it. Meanwhile, fratricide was the rule. Under the fog of war, the Armenians (not really dealt with here) were put to the sword in the 20th century’s first genocide, and British artillery was used in Dublin streets to put down an Irish rising.

Ruthless as they were in the killing of others, the generals were also shockingly profligate and callous when it came to their “own.” In some especially revolting passages, we find Gen. Sir Douglas Haig and his arrogant subordinate Gen. Sir Henry Rawlinson actually complaining when British casualties were too low, and exulting — presumably because enemy losses were deemed comparable — when they moved into the tens of thousands. What this meant in cold terms was the destruction of whole regiments, often comprising (as in the cases of Newfoundland and Ulster) entire communities back home who had volunteered as a body and stayed together in arms. They vanished, in clouds of poison gas, hails of steel splinters and great lakes of sucking mud. Or lay in lines, reminding all observers of mown-down corn, along the barbed wire and machine-gun emplacements against which they had been thrown. Like me, Hochschild has visited the mass graves and their markers, which still lie along the fields of northern France and Belgium, and been overwhelmed by what Wilfred Owen starkly and simply called “the pity of War.” (Owen was to die pointlessly as the guns were falling silent: his mother received the telegram as the church bells were ringing to celebrate the armistice — or better in retrospect to say “fragile cease-fire.”)

We read these stirring yet wrenching accounts, of soldiers setting off to battle accompanied by cheers, and shudder because we know what they do not. We know what is coming, in other words. And coming not only to them. What is really coming, stepping jackbooted over the poisoned ruins of civilized Europe, is the pornographic figure of the Nazi. Again, Hochschild is an acute register. He has read the relevant passages of “Mein Kampf,” in which a gassed and wounded Austrian corporal began to incubate the idea of a ghastly revenge. He notes the increasing anti-Semitism of decaying wartime imperial Germany, with its vile rumors of Jewish cowardice and machination. And he approaches a truly arresting realization: Nazism can perhaps be avoided, but only on condition that German militarism is not too heavily defeated on the battlefield.

This highly unsettling reflection is important above all for American readers. If General Pershing’s fresh and plucky troops had not reached the scene in the closing stages of the bloodbath, universal exhaustion would almost certainly have compelled an earlier armistice, on less savage terms. Without President Wilson’s intervention, the incensed and traumatized French would never have been able to impose terms of humiliation on Germany; the very terms that Hitler was to reverse, by such relentless means, a matter of two decades later. In this light, the great American socialist Eugene V. Debs, who publicly opposed the war and was kept in prison by a vindictive Wilson until long after its ending, looks like a prescient hero. Indeed, so do many of the antiwar militants to whose often-buried record Hochschild has done honor. (Unsentimental to the last, though, he shows that many of them went on to lose or waste their lives on Bolshevism, the other great mutant system to emerge from the abattoir.) This is a book to make one feel deeply and painfully, and also to think hard.