 In the Soviet Union , speaking to his generals in December 1940 , Stalin mentioned Hitler 's references to an attack on the Soviet Union in Mein Kampf and Hitler 's belief that the Red Army would need four years to ready itself . Stalin declared " we must be ready much earlier " and " we will try to delay the war for another two years " . As early as August 1940 , British intelligence had received hints of German plans to attack the Soviets only a week after Hitler informally approved the plans for Barbarossa and warned the Soviet Union accordingly . But Stalin 's distrust of the British led him to ignore their warnings in the belief that they were a trick designed to bring the Soviet Union into the war on their side . He had an ill @-@ founded confidence in the Molotov – Ribbentrop Pact and suspected the British of trying to spread false rumours in order to trigger a war between Germany and the USSR . In early 1941 , Stalin 's own intelligence services and American intelligence gave regular and repeated warnings of an impending German attack . Soviet spy Richard Sorge also gave Stalin the exact German launch date , but Sorge and other informers had previously given different invasion dates that passed peacefully before the actual invasion . Stalin acknowledged the possibility of an attack in general and therefore made significant preparations , but decided not to run the risk of provoking Hitler . 
