If one had asked an informed observer in 1900 where a mass murder of the Jews was likely to take place, he would have pointed without hesitation to the tsarist empire, the scene of many pogroms. If one had asked the same observer where a proletarian revolution was most to be expected, he would have thought of Germany, where the Social Democrats, for all their reformism, were still the largest and best-organised Marxist party in Europe. In reality, of course, these expectations were to be confounded: backward Russia saw the Bolshevik Revolution of 1917, whereas Germany, once a haven for embattled eastern Jews, produced Nazi antisemitism and ultimately the Holocaust. This is the central paradox of the 20th century.

Michael Burleigh's valuable new collection on the Third Reich confronts one part of this paradox directly. "If anyone," he argues, "at the turn of the century was to have predicted the probable location for eruptions of anti-semitism, they would have alighted upon France, Poland, Rumania or Russia, rather than Germany"; and the dustjacket promises to show how the Third Reich "compare(s) with other examples of 20th-century barbarity".

Burleigh insists on the uniqueness of the German experience: he firmly rejects any attempt to "normalise", "historicise" or "relativise" the Third Reich in the context of broader modernisation processes, the development of the welfare state, totalitarianism or the mass murders committed by Soviet Russia. His collection is thus also a belated contribution to the German historians' dispute of the mid-1980s, the Historikerstreit, in which left-liberal historians found themselves pitted against conservatives and centrists who argued, inter alia, that the Nazi crimes needed to be seen in the context of Stalin's murder of the independent peasantry (kulaks) and other opponents of the regime during collectivisation and the purges that followed.

The most useful aspect of the book is that it makes recent research in established fields such as the economy, working-class attitudes and the army accessible to broader audiences. It also introduces new or relatively neglected areas of research such as economic planning and the Holocaust, Gypsies, homosexuals and Burleigh's own field, euthanasia, to which he brings a blend of passion, empathy and empiricism. Ulrich Herbert, for example, takes a refreshing look at working-class racism. He questions the late Tim Mason's view that the proletariat remained sceptical of Nazism. In fact, Herbert argues, the Nazis were successful in "depoliticising" and suborning the working class. By the middle of the war, seduced by military victories and economic benefits, the German working class had become a labour aristocracy of supervisors and mastercraftsmen lording it over droves of foreign workers. Another sacred historiographical cow, this time of the right, is slaughtered by Omer Bartov, who documents the deep involvement of the German army (Wehrmacht), not just the SS, in the war of annihilation against Jews and Bolshevism in the east. Moreover, far from being motivated by "primary" loyalties to old school friends or neighbours, the German army became increasingly atomised, "barbarised" by its experiences and reliant on Nazi ideology to hold it together.

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Perhaps the most stimulating and radical essay in the collection is Gotz Aly's examination of the role of economic and demographic experts - "a careerist, academically trained intelligentsia" - in the planning of the Holocaust. "Expulsions and compulsory" Germanisation, he argues, "were carried out according to calculated economic, as opposed to racial-biological, criteria." These criteria were derived from voodoo-Malthusian theories on rural overpopulation, especially in Poland. Thus "genocide was a means of solving the social question"; and because cheap Jewish labour inhibited investment in efficient new plant, it became "economically more rational to kill the Jews than to put them to work". Far from being economically self-destructive, the murder of the Jews was part of a broader modernising assault on small shopkeepers and artisans. "So-called Aryanisation was therefore primarily a matter of economic rationalisation and concentration, what is now known as 'restructuring'."

But there are problems with Burleigh's approach. His introduction, which is combative even by the robust standards of the subject, is not always consistent. Because he is concerned not to "relativise" the Third Reich, Burleigh denies there is any "intellectually serious comparison to be made between the regimes of Hitler and Honecker or between the Stasi and the Gestapo". Yet in the same chapter, if not in the same breath, he praises Wolfgang Wippermann for highlighting "continuities between treatment of Sinti and Roma Gypsies in the Nazi and postwar periods"; are we to take it then that intellectually serious comparisons may be made between the Third Reich and the Federal Republic but not between the former and the German Democratic Republic? Also, Burleigh objects to common memorials to those murdered in concentration camps before and after 1945, because many of the latter were former camp guards: "a singularly disgraceful aggregation of victims and perpetrators". But many postwar camp inmates in the Soviet zone were bona fide Social Democrats, Christians or trade unionists, guilty of no more than opposition to communism. Is Burleigh not guilty of a little "aggregation" of his own here?

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Some of the new research presented is highly contentious, in particular that of Bartov. His insistence on Wehrmacht complicity in crimes against humanity is undoubtedly correct but not new; his more innovative claim that Nazi ideology held the German army together is rather more controversial. First of all, the sample of soldiers' letters on which much of his argument hinges is taken from unrepresentative Nazi propaganda material. Second, Bartov's thesis does not explain why the only significant renegade officers' movement - the Nationalkomitee Freies Deutschland - arose on the supposedly brainwashed eastern front, while some of the most fanatical Nazis were to be found among U-Boat crews in the North Atlantic, far away from any Bolshevik "subhumans".

Finally, Burleigh's book almost entirely skirts the promised comparison with "other examples of 20th-century barbarity". The exception here is Aly, whose observations are rather subversive of Burleigh's whole project. "The Stalinist population policy of the 1930s - the eradication of the kulaks," he writes, "corresponded in many respects to the policies pursued by the Nazis between 1939 and 1944. Discrimination against minorities, the mobilisation of the rural population, forcible colonisation, slave labour, cultural and linguistic homogenisation and progressive forms of extermination. Both dictatorships pursued strategies of more or less violent modernisation." Aly concludes: "The deed and the crime are unique. However, Auschwitz is part of European as well as German history I a possibility inherent in European civilisation itself." For sheer relativisation that is hard to beat.

A not dissimilar picture emerges from Stanley Payne's A History of Fascism, the fruit of a lifetime's research and reflection. Payne arrives at a "retrodictive" theory of fascism that contains few surprises but is all the more comprehensive for that. Cultural crisis, secularisation, a fragmented political system, a Bolshevik threat, middle-class fears of proletarianisation, economic crisis and imperialist ambitions: these are among the most important preconditions for European fascism. At one level, therefore, the German experience was far from unique. But, as Payne points out, where the Nazis parted company with the fascist mainstream was in their obsessive concern with expansionist war and, of course, "biological racial revolution", that is: genocide.

Payne resembles Aly in his willingness to compare the Nazi and the Stalinist systems. Significantly, he classifies fascism as (only) "one of the two most atrocious political movements of modern times"; the other is plainly Stalinist Russia and its subsequent imitators in Asia. Not only, Payne notes, did Nazis and Bolsheviks often regard each other as birds of a feather, but the common insistence on the leadership principle, one-party dictatorship, political commissars and party armies was striking. Moreover, fascists committed "ghastly mass murder I just as communists murdered millions in the name of an egalitarian Utopia"; and Payne makes a point of mentioning "the tentative [ie, unproven] introduction of gas vans to kill kulaks". In short, "large-scale genocide or mass murder is a prototypical development of the 20th century. The unique Nazi tactic was to modernise the process, to accomplish the mass murder more efficiently and surgically than other great liquidators in Turkey, Russia, or Cambodia." If the uniqueness of the Holocaust lay in planning the extermination of an entire race, the argument runs, Stalinist Russia attempted the physical extermination of an entire class.

Both volumes are primarily concerned with ideology and domestic policies. This is particularly marked in Burleigh's collection, where traditional foreign policy is notable by its absence: homosexuals, Gypsies and women, for example, are given precedence over generals and diplomats. These priorities are reversed by the Austrian-born diplomat Reinhard Spitzy, whose memoirs, excellently translated and abridged by Geoffrey Waddington as How We Squandered the Reich, are now in search of an English-language publisher.

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In conception, Spitzy's book is a conventional apologia: he intends to "justify [him]self and [his] actions"; he wishes to show that he and his contemporaries were "neither scoundrels nor idiots". But he makes no apologies: "People should have the courage to stand by what they have done throughout the course of their lives." Often, Spitzy seems to make apologies for Hitler, too. His recent "postscript" - which is more apologetic and less perceptive than the original memoir - asks provocatively whether Hitler did not "have the right to be guilty". This book is certainly not the place to look for the plight of the Jews. For example, Spitzy portrays the Anschluss only in terms of an emotional homecoming for Hitler and himself; there is no sense that for Austrian Jews this was the beginning of a Via Dolorosa starting with segregation and ending in extermination. Unsurprisingly, the book has given offence, so much so indeed that potential British publishers have tried to persuade Spitzy to tone down his views before publication. This is ultimate historical correctness: a distortion of the documentary record as inadmissible as those perpetrated by hagiographic editors in days gone by. It is also self-defeating: a bowdlerised memoir will be useless to specialists and of no interest to the public.

In practice, the result is far from a conventional apologia. For one thing, Spitzy is at pains to stress that Hitler enjoyed the support of all sections of society: "The phenomenon of the Third Reich was I a German problem through and through." Indeed, there is much in this book that simply endorses existing interpretations of the Third Reich. In common with Alltagsgeschichte, he insists on the relative "normality" of everyday life. His detailed and often comic descriptions of Ribbentrop's struggles with rival paladins such as Goebbels and Goering, and the institutional competition with Wilhelm Bohle's Auslandsorganisation, Alfred Rosenberg's Amt Rosenberg and Konstantin von Neurath's foreign office, confirms the classic theories of the Third Reich as a "polyocracy" of feuding power centres. Spitzy also takes a "functionalist" view of the notorious Blomberg-Fritsch crisis of 1938. Far from being a long-planned coup, he argues, Hitler found himself wrong-footed by an unexpected scandal that he then sought to mask by an impromptu cabinet reshuffle; this is confirmed by recent research on the subject. Finally, Spitzy cites the case of the Khevenhuller-Metsch family - who courageously refused to sell their castle to the SS - as a refutation of "those who insist that it was impossible to say 'no' in the Third Reich, especially to Himmler, without having to fear for one's life in consequence"; this is not so very different from Christopher Browning's argument in Ordinary Men, that one could refuse to participate in genocide and yet live.

But the main focus of Spitzy's memoirs is foreign policy. The root of all evil are the "dictated" settlements of Versailles and (Spitzy being Austrian) St Germain, which made German revanchism inevitable; the French wartime prime minister Clemenceau is thus "the man really responsible for the rise of Hitler". After an adventurous spell as an illegal Austrian Nazi, Spitzy found himself in London as the adviser to the ambassador, Joachim von Ribbentrop. Here the erstwhile revolutionary soon gravitated towards the diplomats of the old school alienated by Ribbentrop's insensitive and counter-productive posturing towards Britain. Indeed, it was the catastrophic collapse of Anglo-German relations - not any misgivings about domestic policy - that brought on Spitzy's first serious and sustained doubts about the regime. For although he is scathing about "Eden and the Vansittart-Churchill clique", Spitzy leaves no doubt that the second world war was "wilfully and recklessly unleashed by Hitler and the Ribbentrop couple". He later became a sympathiser with the German resistance, and at times even an active resister, though his well-founded doubts about the competence and discretion of the conspirators saved him from being directly implicated in the July plot of 1944.

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By the time Spitzy's memoirs were penned in 1946, the paradoxical effects of Nazi policy were painfully evident. Far from restoring the old German empire with British help and uniting Europe against Bolshevism, Hitler had succeeded in uniting Britain and the world against Germany. The Reich was partitioned; Soviet power advanced into the heart of central Europe.

But the 20th century had yet to run its paradoxical course. Some 50 years later, Spitzy's postscript to the fourth German edition (1994) reveals a changed attitude towards Britain; spurned affection gives way to pity. From being one of the big three at Potsdam, Britain has declined into a cantankerous (Euro-)sceptical isle set in a tarnished sea of unsellable agricultural produce. Whereas, Spitzy notes without Schadenfreude but with brutal matter-of-factness, "Germany has been reunited ... Britain, on the other hand, has shrunk to the status of a Middle Power".

Brendan Simms is a fellow and director of studies in history, Peterhouse, Cambridge.