Why do you think Stalin decided to form a pact with Hitler in August 1939?

On this day in 1939, Germany and the Soviet Union sign a non-aggression pact, stunning the world, given their diametrically opposed ideologies. But the dictators were, despite appearances, both playing to their own political needs.

After Nazi Germany’s invasion of Czechoslovakia, Britain had to decide to what extent it would intervene should Hitler continue German expansion. Prime Minister Neville Chamberlain, at first indifferent to Hitler’s capture of the Sudetenland, the German-speaking area of Czechoslovakia, suddenly snapped to life when Poland became threatened. He made it plain that Britain would be obliged to come to the aid of Poland in the event of German invasion. But he wanted, and needed, an ally. The only power large enough to stop Hitler, and with a vested interest in doing so, was the Soviet Union. But Stalin was cool to Britain after its effort to create a political alliance with Britain and France against Germany had been rebuffed a year earlier. Plus, Poland’s leaders were less than thrilled with the prospect of Russia becoming its guardian; to them, it was simply occupation by another monstrous regime.

Hitler believed that Britain would never take him on alone, so he decided to swallow his fear and loathing of communism and cozy up to the Soviet dictator, thereby pulling the rug out from the British initiative. Both sides were extremely suspicious of the other, trying to discern ulterior motives. But Hitler was in a hurry; he knew if he was to invade Poland it had to be done quickly, before the West could create a unified front. Agreeing basically to carve up parts of Eastern Europe-and leave each other alone in the process-Hitler’s foreign minister, Joachim von Ribbentrop, flew to Moscow and signed the non-aggression pact with his Soviet counterpart, V. M. Molotov (which is why the pact is often referred to as the Ribbentrop-Molotov Pact). Supporters of bolshevism around the world had their heretofore romantic view of "international socialism" ruined; they were outraged that Stalin would enter into any kind of league with the fascist dictator.

But once Poland was German-occupied territory, the alliance would not last for long.