"You know that we have to exterminate these vermin if we and our families are to live. We must go on to the end if civilisation is to survive." This could be Goebbels talking about the Jews, but in fact it is General Thomas Blamey urging Australian troops to take revenge on the Japanese for atrocities such as rape, torture, murder and cannibalism. And this was not the only time when allied attitudes duplicated those of the Nazis. US marines went into combat on Iwo Jima with the words "Rodent Exterminator" stencilled on their helmets.

Michael Burleigh, who cites these instances in his magnificent moral survey of the second world war, is not suggesting any ethical equivalence between the democracies and the dictatorships (one of which was, of course, an ally). He is contemptuous of those who try to equate, say, Auschwitz and Hiroshima: whereas the Jews posed no threat to Germany, the atom bomb was used against a foe that was not only still fighting but (a point seldom made) killing more than 100,000 Asian slave labourers each month. No, what Burleigh does is to chart the prevailing moral sentiment of the belligerent states and their leaders and to examine how this changed under the pressure of war. He avoids moralising, "which is to morality what artiness is to art". His book is a moral map, not a moral compass.

Yet he writes with a marvellous trenchancy that sometimes becomes savage indignation. Burleigh's account of Neville Chamberlain's foreign secretary, Lord Halifax, usually treated with respect as "the Holy Fox", is splendidly scathing. "His memoirs describe with pious, self-deprecating smugness his smooth ascent, via Eton, All Souls and Delhi, where he was viceroy, all achieved through luck and nepotism, and padded with the usual tedious Oxbridge legends of deaf college porters and solecisms about handling the port which make Englishmen seem like retarded bores."

Burleigh is especially caustic about Hannah Arendt's oft-quoted remark concerning the "banality of evil". Referring to Adolf Eichmann, it implies that those involved in the final solution were unimaginative clerks. Actually Eichmann was a thug as well as a bureaucrat, who screamed at Jews: "Pig, stand against the wall when you address me." Furthermore, Burleigh reminds us, the Holocaust was not just a clanking process of industrialised homicide. Some 2.9 million Jews were murdered by men standing a few feet away from them. Hardly anyone was compelled to kill or punished for refusing since, as one SS infantryman testified, "there were always enough volunteers".

The explanation for this, Burleigh argues, was that the perpetrators retained a sense of morality. Victims of a pseudo-scientific belief in the inequality of human races, they felt engaged in a historic mission to purge the world of Untermenschen. Thus Himmler could talk about the decency and heroism of those who undertook the unpleasant task. Similar considerations governed the conduct of German forces in the Soviet Union and resulted in the death of 3.3 million Russian prisoners of war.

Yet perhaps this is not the whole story. As Burleigh acknowledges, most Germans nominally subscribed to Christian values  unlike the Japanese, who had a "genuine excuse" for savagery since they possessed no moral code overriding their duty to the divine emperor. Furthermore, even the SS death squads were appalled by the bloody work of their Romanian auxiliaries on the eastern front  repaid in kind, needless to say, by the Red Army. And at home many Germans concluded that the allied bombing raids were a revenge for the monstrous crime committed against the Jews, which suggested widespread guilt and awareness.

The RAF's attack on German cities began unintentionally, because after Dunkirk Churchill had no other weapon, because bombers could not hit targets with precision and because the public demanded retaliation for the blitz. The politicians did not acknowledge that they were aiming at civilians, though Churchill himself veered between acute anxiety over the use of terror tactics and an intense desire to smash the Third Reich to smithereens. Burleigh admires Air Marshal "Bomber" Harris for his forthright assault on venal arms manufacturers (assisted by corrupt ex-inmates of the Ministry of Defence) and his candour about the bombing campaign. It was not a war crime, Burleigh says. But he admits that many regard it as "morally repugnant" and he takes seriously Bishop Bell's claim that it detracted from the fundamental justice of the allied cause.

Despite its subtitle, this book is not a history of the second world war; far from being comprehensive, it contains, for example, almost nothing about the war at sea and concentrates largely on Hitler, "'our' monster". What Burleigh has produced is a long, brilliant, original, opinionated and scholarly meditation on morality as it appeared, or failed to appear, during the last global conflict. A few stylistic and factual slips apart, it is virtually impeccable. Seldom has a study of the past combined such erudition with such exuberance.

Piers Brendon's The Decline and Fall of the British Empire

The ethics of foreign policy is big business in Anglo-Saxon cultures. For self-evident reasons its practitioners are increasingly people who have never witnessed the death of a soldier, or been involved in the decision that led to it. Not that this invalidates their work; it simply adds weight to the writings on morality of previous generations, far more of whom had fought in wars, or borne responsibilities on which lives depended.

This comes to mind when reading Michael Burleigh's imposing book, because he has scrupulously avoided the abiding sins of the ethics brigade: abstracting, decontextualising and isolating the morality of an action as if it were some specific gene, whose health or toxicity we don our white coats to examine. So this is no arid re-run of debates about the Melian dialogue, or the guilt of entire peoples. Moral Combat deals with morality as something inseparable from life.

Comparisons between communism and fascism, inevitably, are an overarching theme, but rather than banging on about Reds and Nazis, même combat, he allows the truth to emerge from his narrative. And because the book is narrative, rather than moralistic theory, we read it.

Concentration camps were institutionalised in both countries, first in Russia then, on a far smaller scale in Hitlerite Germany, though the links are stark. It was news to me that prior to their invasion German planners had toyed with the idea of dispatching Jews to work/die in Stalin's Arctic gulag, where the communist equivalent of Arbeit Macht Frei over the gates was "Labour is a Matter of Honour, Courage and Heroism". On style, cynicism and concision at least, the Nazis won.

Burleigh is a master of significant detail unearthed in microscopic research. His account of the grumbles of SS slaughtermen about being spattered with brains and bits of skull reminded me that Russian NKVD killers who made the same complaint were given extra vodka and eau de Cologne by way of compensation. A small point, perhaps, but something that brings the reality of atrocity alive to a generation jaded by Nazi horror films, and who have seen few, if any, about the gulag, and one that makes it that bit harder to argue that there can be no comparison between the systems. Either way, Burleigh reminds us, you are dead, your brains on the smock of a drunken, psychotic thug.

The book also points revealingly to non-parallels between the twin totalitarians. In Germany there were muffled protests against the inhumanity of the campaign in the east, but military men of conscience  these things are relative  were often simply retired. In Russia there is little evidence of such soul-searching, not only, one suspects, because Stalin would have had the quibblers shot.

Nor in Germany was there anything remotely equivalent in scale to the show trials and party purges that cost 750,000 Russian lives, or the millions starved to death through the dogma of collectivisation. You could argue Hitler made up the numbers in his extermination of various breeds of Untermenschen. But then that would be to forget Dr Johnson's injunction against distinguishing between fleas and lice, which Burleigh, preferring to let facts speak, wisely avoids.

His conclusion is sane and simple: reducing individuals to culpable groups, and seeing the solutions to the problems of mankind in their extermination, is the ultimate crime, whether perpetrated by Mao, Pol Pot, Stalin or Hitler. End of discussion, I should have thought, though for diehard romantics, notably on the left, it never is.

Another myth Burleigh's book cumulatively destroys is that Nazi barbarism was foisted on a decent, reluctant soldiery by crazed ideologues. Though individuals did find ways to dodge their race-annihilation duties, "there were always enough volunteers". And German troops dashed Jewish babies against walls not just through drunkenness, or because their Ur-racism had been officiously stoked to a fury, but "because they felt like it".

The book does not shrink from moral disorder in the Allied camp. In public Churchill said the obliteration of Dresden he had specifically approved was "a bombing too far". Truman swallowed the fiction that the atom bomb destined for Nagasaki would aim at military and industrial targets.

As always the colour and power of Burleigh's prose is outstanding. When it comes to evoking the smell of a place he is far and away our best history writer. A passing example: in Soviet Russia innocents were dispatched to the gulag "in slow trains that clanked to the peripheries of empire".

This is no comfortable, reflective read, but the bloodiest and most wrenching account of the war I can recall. Morally speaking, the reader, however gentle, begins to feel a little blood-spattered too, because the problem with inhumanity on this scale is that it was practised, not by abstract systems, but by human beings. What more sombre message could there be?

George Walden is a former diplomat and MP. His book China: A Wolf in the World? is published in paperback next month by Gibson Square.