Two weeks before the war, Stalin proposed the pact, but neither the British nor French delegations accepted
This article originally appeared in The Telegraph
Papers which were kept secret for almost 70 years show that the Soviet Union proposed sending a powerful military force in an effort to entice Britain and France into an anti-Nazi alliance.
Such an agreement could have changed the course of 20th century history, preventing Hitler's pact with Stalin which gave him free rein to go to war with Germany's other neighbours.
The offer of a military force to help contain Hitler was made by a senior Soviet military delegation at a Kremlin meeting with senior British and French officers, two weeks before war broke out in 1939.
The new documents, copies of which have been seen by The Sunday Telegraph, show the vast numbers of infantry, artillery and airborne forces which Stalin's generals said could be dispatched, if Polish objections to the Red Army crossing its territory could first be overcome.
But the British and French side - briefed by their governments to talk, but not authorised to commit to binding deals - did not respond to the Soviet offer, made on August 15, 1939. Instead, Stalin turned to Germany, signing the notorious non-aggression treaty with Hitler barely a week later.
The Molotov-Ribbentrop Pact, named after the foreign secretaries of the two countries, came on August 23 - just a week before Nazi Germany attacked Poland, thereby sparking the outbreak of the war. But it would never have happened if Stalin's offer of a western alliance had been accepted, according to retired Russian foreign intelligence service Major General Lev Sotskov, who sorted the 700 pages of declassified documents.
"This was the final chance to slay the wolf, even after [British Conservative prime minister Neville] Chamberlain and the French had given up Czechoslovakia to German aggression the previous year in the Munich Agreement," said Gen Sotskov, 75.
The Soviet offer - made by war minister Marshall Klementi Voroshilov and Red Army chief of general staff Boris Shaposhnikov - would have put up to 120 infantry divisions (each with some 19,000 troops), 16 cavalry divisions, 5,000 heavy artillery pieces, 9,500 tanks and up to 5,500 fighter aircraft and bombers on Germany's borders in the event of war in the west, declassified minutes of the meeting show.
But Admiral Sir Reginald Drax, who lead the British delegation, told his Soviet counterparts that he authorised only to talk, not to make deals.
"Had the British, French and their European ally Poland, taken this offer seriously then together we could have put some 300 or more divisions into the field on two fronts against Germany - double the number Hitler had at the time," said Gen Sotskov, who joined the Soviet intelligence service in 1956. "This was a chance to save the world or at least stop the wolf in its tracks."
When asked what forces Britain itself could deploy in the west against possible Nazi aggression, Admiral Drax said there were just 16 combat ready divisions, leaving the Soviets bewildered by Britain's lack of preparation for the looming conflict.
The Soviet attempt to secure an anti-Nazi alliance involving the British and the French is well known. But the extent to which Moscow was prepared to go has never before been revealed.
Simon Sebag Montefiore, best selling author of Young Stalin and Stalin: The Court of The Red Tsar, said it was apparent there were details in the declassified documents that were not known to western historians.
"The detail of Stalin's offer underlines what is known; that the British and French may have lost a colossal opportunity in 1939 to prevent the German aggression which unleashed the Second World War. It shows that Stalin may have been more serious than we realised in offering this alliance."
Professor Donald Cameron Watt, author of How War Came - widely seen as the definitive account of the last 12 months before war began - said the details were new, but said he was sceptical about the claim that they were spelled out during the meetings.
"There was no mention of this in any of the three contemporaneous diaries, two British and one French - including that of Drax," he said. "I don't myself believe the Russians were serious."
The declassified archives - which cover the period from early 1938 until the outbreak of war in September 1939 - reveal that the Kremlin had known of the unprecedented pressure Britain and France put on Czechoslovakia to appease Hitler by surrendering the ethnic German Sudetenland region in 1938.
"At every stage of the appeasement process, from the earliest top secret meetings between the British and French, we understood exactly and in detail what was going on," Gen Sotskov said.
"It was clear that appeasement would not stop with Czechoslovakia's surrender of the Sudetenland and that neither the British nor the French would lift a finger when Hitler dismembered the rest of the country."
Stalin's sources, Gen Sotskov says, were Soviet foreign intelligence agents in Europe, but not London. "The documents do not reveal precisely who the agents were, but they were probably in Paris or Rome."
Shortly before the notorious Munich Agreement of 1938 - in which Neville Chamberlain, the British prime minister, effectively gave Hitler the go-ahead to annexe the Sudetenland - Czechoslovakia's President Eduard Benes was told in no uncertain terms not to invoke his country's military treaty with the Soviet Union in the face of further German aggression.
"Chamberlain knew that Czechoslovakia had been given up for lost the day he returned from Munich in September 1938 waving a piece of paper with Hitler's signature on it," Gen Sotksov said.
The top secret discussions between the Anglo-French military delegation and the Soviets in August 1939 - five months after the Nazis marched into Czechoslovakia - suggest both desperation and impotence of the western powers in the face of Nazi aggression.
Poland, whose territory the vast Russian army would have had to cross to confront Germany, was firmly against such an alliance. Britain was doubtful about the efficacy of any Soviet forces because only the previous year, Stalin had purged thousands of top Red Army commanders.
The documents will be used by Russian historians to help explain and justify Stalin's controversial pact with Hitler, which remains infamous as an example of diplomatic expediency.
"It was clear that the Soviet Union stood alone and had to turn to Germany and sign a non-aggression pact to gain some time to prepare ourselves for the conflict that was clearly coming," said Gen Sotskov.
A desperate attempt by the French on August 21 to revive the talks was rebuffed, as secret Soviet-Nazi talks were already well advanced.
It was only two years later, following Hitler's Blitzkreig attack on Russia in June 1941, that the alliance with the West which Stalin had sought finally came about - by which time France, Poland and much of the rest of Europe were already under German occupation.