Great Britain’s cowardly and short sighted abandonment of Czechoslovakia in 1938, followed by the secret and now infamous Molotov-Ribbentrop Pact of 1939, put Europe on a hair trigger towards war. Stalin and Hitler were willing to destroy nations and murder millions of people in pursuit of their parallel and conflicting goals of controlling the grainlands of Poland, Ukraine, and adjacent lands and peoples. Hitler turning his aggression against Russia, dooming so many (the terrifying number of deaths quoted in the records referred to upthread crucially disproportionately include huge Ukrainian rates of death that Stalin never gave a damn about).
The short sighted cupidity, cowardice and fecklessness of Great Britain, France, and the USA populace (FDR was a different matter), combined with Russian/German aggressions, brought on the horrors.