Bloodlands: Europe Between Hitler and Stalin. By Timothy Snyder. Basic Books; 524 pages; $29.95. Bodley Head; £20. Buy from Amazon.com, Amazon.co.uk

IN THE middle of the 20th century Europe's two totalitarian empires, Nazi Germany and Stalin's Soviet Union, killed 14m non-combatants, in peacetime and in war. The who, why, when, where and how of these mass murders is the subject of a gripping and comprehensive new book by Timothy Snyder of Yale University.

The term coined in the book's title encapsulates the thesis. The “bloodlands” are the stretch of territory from the Baltic to the Black Sea where Europe's most murderous regimes did their most murderous work. The bloodlands were caught between two fiendish projects: Adolf Hitler's ideas of racial supremacy and eastern expansion, and the Soviet Union's desire to remake society according to the communist template. That meant shooting, starving and gassing those who didn't fit in. Just as Stalin blamed the peasants for the failure of collectivisation, Hitler blamed the Jews for his military failures in the east. As Mr Snyder argues, “Hitler and Stalin thus shared a certain politics of tyranny: they brought about catastrophes, blamed the enemy of their choice, and then used the death of millions to make the case that their policies were necessary or desirable. Each of them had a transformative Utopia, a group to be blamed when its realisation proved impossible, and then a policy of mass murder that could be proclaimed as a kind of ersatz victory.”

Mr Snyder's book is revisionist history of the best kind: in spare, closely argued prose, with meticulous use of statistics, he makes the reader rethink some of the best-known episodes in Europe's modern history. For those who are wedded to the simplistic schoolbook notions that the Hitlerites were the mass murderers and the Soviets the liberators, or that the killing started in 1939 and ended in 1945, Mr Snyder's theses will be thought-provoking or shocking. Even those who pride themselves on knowing their history will find themselves repeatedly brought up short by his insights, contrasts and comparisons. Some ghastly but well-known episodes recede; others emerge from the shadows.

Sometimes the memories are faded because so few were left to remember. Those who suffered horribly but lived to tell the tale naturally get a better hearing than the millions in unmarked graves. Mr Snyder's book straightens the record in favour of the voiceless and forgotten.

He starts with the 3.3m in Soviet Ukraine who died in the famine of 1933 that followed Stalin's ruthlessly destructive collectivisation. He goes on to mark the 250,000-odd Soviet citizens, chiefly Poles, shot because of their ethnicity in the purges of 1937-38. Sometimes the NKVD simply picked Polish-sounding names from the telephone directory, or arrested en masse all those attending a Polish church service.

Some stories remained untold because they were inconvenient. About as many people died in the German bombing of Warsaw in 1939 as in the allied bombing of Dresden in 1945. Post-war Poland was in no state to gain recognition for that. The Nazi-Soviet alliance of August 1939 was “cemented in blood”, Stalin said approvingly. Few wanted to remember that two years later, when the Germans invaded the Soviet Union in Operation Barbarossa. The Western allies did little to stop the Holocaust. Few wanted reminding that the only government that took direct action to help the Jews was the Polish one: seven of the first eight operations conducted in Warsaw by the underground Polish Home Army were in support of the ghetto uprising. (After the war, the Communist authorities executed as “fascists” Polish soldiers who had helped the Jews.)

Stalin regarded all Soviet prisoners-of-war as traitors. Their German captors starved them to death in their millions; nobody dared mourn them. The Holocaust, too, did not fit into Soviet historiography, especially as post-war anti-Semitism intensified (“Every Jew is a nationalist and an agent of American intelligence,” Stalin said in 1952). Memorials to murdered Jews carried not the Star of David but the five-pointed Soviet one, and referred blandly to “Soviet citizens” or “victims of fascism”.

Many of the stories in the book are already known as national or ethnic tragedies. Poles focus on the Warsaw uprising; Jews on Auschwitz; Russians on the siege of Leningrad; Ukrainians on the great famine. Mr Snyder's book weaves the stories together, explaining how the horrors interacted and reinforced each other. Hitler learnt a lot from Stalin, and vice versa.

Mr Snyder shifts the usual geographical focus away from the perpetrator countries to the places where they first colluded and then collided. Germany and Russia (and Germans and Russians) mostly fared better, or less horribly, than the places in between (there were more Jews in the Polish city of Lodz alone than in Berlin and Vienna combined). No corner of what are now Belarus and Ukraine was spared. Much of Germany and even more of Russia was unscathed, at least physically, by war.

He also corrects exaggerations, misapprehensions and simplifications. The bestial treatment of slave labourers in concentration camps, and the use of gas chambers, are commonly seen as the epitomes of Nazi persecution. But the Germans also shot and starved millions of people, as well as gassed and worked them to death. In just a few days in 1941, the Nazis shot more Jews in the east than they had inmates in all their concentration camps.

“Bloodlands” has aroused fierce criticism from those who believe that the Soviet Union, for all its flaws, cannot be compared to the Third Reich, which pioneered ethnic genocide. Doing this, the critics argue, legitimises ultranationalists in eastern Europe who downplay the Holocaust, exaggerate their own suffering—and dodge guilt for their own collaboration with Hitler's executioners.

That argument is powerful but unfair. Many people say stupid things about history. Mr Snyder is not one. He does not challenge the Holocaust's central place in 20th-century history. Nor does he overlook Soviet suffering at the hands of Hitler or the heroism of the soldiers who destroyed the Third Reich. But he makes a point that needs reinforcement, not least in Russia where public opinion and officialdom both retain a soft spot for Stalin's wartime leadership. The Soviet Union's ethnic murders predated Nazi Germany's. Stalin was not directly responsible for the Holocaust, but his pact with the Nazis paved the way for Hitler's killing of Jews in the east.

Mr Snyder's scrupulous and nuanced book steers clear of the sterile, sloganising exchanges about whether Stalin was as bad as Hitler, or whether Soviet mass murder in Ukraine or elsewhere is a moral equivalent of the Nazis' extermination of the Jews. What it does do, admirably, is to explain and record. Both totalitarian empires turned human beings into statistics, and their deaths into a necessary step towards a better future. Mr Snyder's book explains, with sympathy, fairness and insight, how that happened, and to whom. Just don't read it before bedtime.