After the Molotov-Ribbentrop Non Aggression Pact in August 1939, and the dual invasion of Poland the next month, National Socialist Germany and Soviet Communist Russia were de facto allies, a state of affairs Hitler was happy to promulgate. His reason: Operation Barbarossa, the German invasion of the Soviet Union was set to begin on 22 June 1941. On 28 May, Hitler and his generals finished the final conditions check. With the Polish mud finally drying, Hitler approved the last mass of German troops to move in a carefully orchestrated and extremely efficient manner into their staging areas along the Soviet border. They would certainly be detected. That’s just three weeks to move more than a million more men (joining two million already there) and their equipment to the border.
Although Stalin considered Hitler a “Brother in Socialism” (his words), he was not naive. He’d read Mein Kampf. In an ideology where might makes right, only one flavor of socialism could be dominant. Conflict with Nazi Germany was inevitable. Stalin just believed that Hitler was not be foolish enough to start a war with the Soviet Union before finishing off Great Britain (which was fully supported by Goebbels’ propaganda efforts). Hitler took seven months to prepare for the invasion of France, a significantly smaller endeavor than invading the Soviet Union, and would need at least a year to consolidate the British isles after the expected summer invasion of England (again supported by Goebbels). That meant an invasion of the Soviet Union in 1943 at the earliest, with 1944 more likely. To Stalin, any information to the contrary was just part of a British plot to get the USSR into the war that they were clearly losing.
Stalin was right, but that didn’t mean the information was incorrect. The British had been passing select Ultra intercepts to the Soviets for months detailing Operation Barbarossa. However, Stalin refused to believe Hitler would betray him in 1941, despite information to the contrary from American and British sources, and Soviet spies and sympathizers in Germany. There were some limited mobilizations that spring but it was mostly done behind Stalin’s back by Georgy Zhukov, one of only two marshals to survive Stalin’s still ongoing Great Purge. On 21 May, during a Central Committee meeting, the head of Soviet intelligence, General Ivan Proskurov, argued with Stalin that a German invasion was imminent. After the meeting, Stalin had him taken out back and shot.
There was no more talk of an invasion for the next four weeks.