That Germany was very likely going to invade Poland was hardly a secret. Why else would the USSR have two weeks before offered Britain & France to send in a million of its troops to kill the baby while it was still in the crib? And why did they wait until that offer was refused before signing a non-aggression pact with Germany, a week before Germany invaded?

“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 [Major General] Lev 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.”

“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.

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.

The USSR had known for years that Nazi Germany was going to eventually try to destroy it. The Anti-Comintern pact had been signed six years before. Destroying communism was one of Nazi Germany’s primary goals, which is why first they came for the Communists. The USSR agreed to the Pact in order to buy itself time to built up its military for the inevitable invasion attempt by Germany.