Editor's note: Timothy Snyder's work on the mass killings, both before and during the second world war, in eastern Europe – the territories first divided by treaty and then trampled by conquest and reconquest – has shifted the emphasis of our understanding of the second world war and the Holocaust. In the context of recent debates about the "double genocide" thesis in eastern Europe, Snyder's approach has a particular relevancy to contemporary concerns. Here, he outlines the general theme of his new book, Bloodlands: Europe Between Hitler and Stalin. Two response articles will follow this week.

It is confoundingly hard to remember that the second world war began with an alliance between Hitler and Stalin. In August 1939, Berlin and Moscow signed a non-aggression pact, including a secret protocol in which they divided the lands between Germany and the USSR into spheres of influence. The next month, the Wehrmacht and the Red Army both invaded Poland, met at a demarcation line, and arranged a joint victory parade. Seventy-one years ago today, on 28 September 1939, the Polish campaign complete, the two powers signed a treaty on borders and friendship, finalising their division of Poland and providing for future economic cooperation.

We cannot, of course, know what would have happened had Stalin refused Hitler's overtures and declined to enable Hitler's empire in summer 1939. What we do know is that the worst war in history, with its battles and atrocities, with its starvation and its Holocaust, began this way and not another.

In summer 1939, Hitler wanted a war, and Stalin wanted a truce. Stalin's great fear in the 1930s was encirclement by a coalition of Germany, Poland, and Japan. This led him to have more than 100,000 Soviet citizens, most of them members of the Polish national minority, shot on false charges of espionage for Poland in 1937 and 1938. This was the greatest campaign of ethnic executions in the Europe of the 1930s.

When Warsaw resisted Hitler's demands for Polish territory in spring and summer 1939, Stalin saw an opportunity to resolve his Polish problem entirely. The rapprochement with Germany not only led to the elimination of Poland as a state, it also drove Japan and Germany apart. Tokyo saw itself supplanted by Moscow as Hitler's closest ally.

The spectre of encirclement dissolved. Thus the Nazi-Soviet alliance of 1939 might seem to have been a brilliant stroke by Stalin. But in his crafty diplomacy, he underestimated the most obvious threat: the German invasion of the Soviet Union, which followed in June 1941.

In the meantime, the Soviet Union supplied Nazi Germany with grain and oil as the Wehrmacht invaded Denmark, Norway, Belgium, France, the Netherlands and Luxembourg. Soviet supplies made it impossible for the Royal Navy to effectively blockade Germany during the Battle of Britain. The Red Army also moved west, invading Finland, occupying part of Romania, and annexing the three Baltic States.

Between 1939 and 1941, as they jointly occupied Poland, Moscow and Berlin each sought to destroy its national elites. Despite differences in ideology, the Nazis and Soviets followed strikingly similar policies in Poland. The Soviets deported about 400,000 Polish citizens to Kazakhstan and Siberia, the Germans a similar number to make room for German farmers. Together, the two powers executed tens of thousands of Polish civilians, many of them educated people. The demographic profiling was similar enough that, in some cases, the Germans murdered one sibling and the Soviets another.

When Germany invaded the Soviet Union in June 1941, Stalin reversed alliances, beginning the history that is most comfortable to remember in Russia, Britain and the United States: of the common struggle against Hitler. But for the lands and people between Berlin and Moscow, the change in alliances meant the accumulation of horrific and destructive power. The Soviet Union and Nazi Germany as allies were destructive enough; Nazi Germany at war with the Soviet Union was more destructive still.

The region of Europe most touched by the war was triply-occupied eastern Poland: first, by the Soviets; then, by the Germans; then, by the Soviets again. It was here that the Soviet NKVD made more arrests than in the entire remainder of the Soviet Union in 1940, and here where the Holocaust began in 1941. Entering the lands that they had conceded to Stalin in 1939, the Germans used NKVD crimes as a propaganda justification for the bloody massacres of Jews in summer 1941, in which Lithuanians, Ukrainians, Poles and others took part. The Germans killed a million Jews east of the line established by the treaty on borders of friendship. Then, they turned their attention to the Jews of Poland and Europe.

Because the war began the way it did, its end could not bring closure. Jews could not help but see the return of Soviet power as a liberation. Soviet policy was not especially friendly to Jews, but it was obviously better than a Holocaust. For many others – above all, Poles and Balts – the return of Soviet power at war's end could hardly seem like liberation, since the war itself had begun with a Soviet invasion.

A second line of division in memory lay between east and west. In western Europe, occupation meant German occupation, real or threatened. In eastern Europe, it meant double or triple occupation by both the Germans and the Soviets. The major atrocities of both Nazis and Stalinists were committed in the lands between; occupation by both Germany and the Soviet Union was worse than occupation by Germany alone.

These divisions in memory, between Jews and non-Jews, and between east and west, are reinforced each May, as we commemorate the war's end. Overcoming them requires that we recall that war began, one September, with friendship between Nazi Germany and the Soviet Union.

• Read Efraim Zuroff's response here

