On Saturday 21 June 1941, the German panzer commander Heinz Guderian visited forward positions overlooking the Russian fortress at Brest-Litovsk. Built on the confluence of the Bug and Muchawiec, the town consisted of four islands, ringed by a high earth rampart. Soviet defences along the river stood unattended. Within the fortress, Red Army soldiers drilled to the tunes of a military band. The town was usually garrisoned by 8,000 soldiers, but this weekend the force stood at 3,500 because of leave or assignments elsewhere. At midnight, the Berlin–Moscow express train crossed the Bug in the direction of Brest, with a Soviet grain train rumbling over the frontier in the opposite direction somewhat later. German deserters who relayed the information that an attack was imminent were ignored, and in some cases shot.

In fact, warnings that something grave was afoot had been flowing in for months. Since February, the Soviet ambassador in Berlin, Dekanozov, had reported to Molotov on German preparations for war. Molotov probably downplayed the threat because of his role in making the 1939 Non-Aggression Pact. Careerism, hierarchy and the caution endemic among the Soviet Establishment meant that Dekanozov did not express his fears directly to Stalin. Soviet foreign intelligence services also presented confused and confusing impressions. Agents in Berlin and Tokyo, or from various branches of Soviet secret intelligence, gave conflicting dates for a future invasion.