In the fourth of a series of articles marking the outbreak of World War II 70 years ago, the BBC Russian Service's Artyom Krechetnikov assesses Soviet leader Joseph Stalin's motivations behind the 1939 Soviet-Nazi pact.
Stalin felt a German defeat would delay the global spread of Communism
Soviet government documents released since the USSR's collapse give us a clear idea of what drove Stalin's thinking in concluding the non-aggression treaty - the Molotov-Ribbentrop Pact - with Nazi Germany.
On 19 August 1939, just days before the agreement was signed in Moscow, in a speech to a hastily-convened session of the Politburo, Stalin said the "question of war and peace is entering a decisive phase".
He predicted that the outcome would depend entirely on whichever strategic position the USSR decided to adopt.
Should the Soviet Union form an alliance with France and Britain, he opined, Germany would be forced to abandon its territorial demands on Poland.
This, Stalin suggested, would avoid the threat of imminent war, but it would make "the subsequent development of events dangerous for the Soviet Union".
Should the USSR sign a treaty with Germany, Stalin suggested, Berlin would "undoubtedly attack Poland, leading to a war with the inevitable involvement of France and England".
Looking ahead, Stalin suggested that "under these circumstances, we, finding ourselves in a beneficial situation, can simply await our turn [to extract maximum advantage]".
What is clear is that Stalin not only appeared unconcerned about the prospect of an attack from Nazi Germany, he actually considered such an attack impossible.
"Our aim is to ensure Germany can continue to fight for as long as possible, in order to exhaust and ruin England and France," he said. "They must not be in a condition to rout Germany.
"Our position is thus clear
remaining neutral, we aid Germany economically, with raw materials and foodstuffs. It is important for us that the war continues as long as possible, in order that both sides exhaust their forces."
Many western historians believe that the Anglo-French security guarantees given to Poland effectively turned Stalin into the arbiter of Europe.
On 3 May 1939, Stalin replaced the pro-Western, Jewish Foreign Minister Litvinov, with Vyacheslav Molotov. It was a strong signal that he wanted to improve relations with the Nazis.
Official Russian history asserts that Stalin believed that Germany, even if it were to emerge from war as a victor, would be so exhausted that it would be unable to wage war with the USSR for at least a decade.
The Molotov-Ribbentrop Pact drew unequivocal criticism from Communists outside the USSR.
Stalin invited the head of the Comintern, the international Communist organisation founded in Moscow, to explain his thinking.
"Hitler does not understand or want this, but he is undermining the capitalist system," he said. "What we can do is manoeuvre around the two sides, push one of the sides to attack the other."
In a written note to foreign Communist parties, Stalin asserted: "The salvation of English-French imperialism would be a violation of Communist principles. These principles in no way exclude a temporary agreement with our common enemy, Fascism."
So was there an alternative?
In the spring and summer of 1939, Stalin could have forged an alliance with Western democracies. Such a move may have prevented a world war, with Europe's borders remaining unchanged.
The problem with this, for Stalin, was that it would have delayed what he viewed as the "final global victory of Communism" for an indeterminable period.
Stalin's actions and deeds made it clear that he could not conceive a protracted period of "peaceful co-existence", the notion that came to determine the Soviet Union's policy towards the capitalist world after Stalin's death.
Stalin and Hitler were united by their desire to destroy the old world order, and to rebuild it as they wanted.
Arguably, this made Soviet-Nazi friendship as inevitable as was its rapid, explosive end.