斯大林在1939年春天为了向希特勒示好（原本根据Molotov-Ribbentrop秘密协定是在夏天 ） ， 保证在纳粹对西欧进攻中，其后方将不会受到东面攻击，也就是说将西欧完全留给希特勒为所欲为。
When Stalin offered Hitler his friendship in the spring of 1939 – formally concluded that summer in the Molotov-Ribbentrop Pact – Nazi aggression was assured of not being knifed in the back from the East and so was left with free hands to do as Hitler pleased in the West.