Page "Operation Barbarossa" Paragraph 30

Spy Dr. Richard Sorge gave Stalin the exact German launch date ; Swedish cryptanalysts led by Arne Beurling also knew the date beforehand, but Sorge and other informers ( e. g. from Berlin Police dept.

) had previously given different invasion dates which passed peacefully before the actual invasion.

In addition, British intelligence gathering information through Ultra warned the Soviet Union of impending invasion several months prior to 22 June 1941.