Roessler's first major contribution to Soviet intelligence came in May 1941 when he was able to deliver details of Operation Barbarossa, Germany's impending invasion of the Soviet Union. Though his warning was initially ignored - as Soviet intelligence had received multiple false alarms about an impending German invasion - Roessler's dates eventually proved accurate. Following the invasion, in June 1941, Lucy was regarded as a VYRDO source, "i.e." of the highest importance, and to be transmitted immediately. Over the next two years "Lucy" was able to supply the Soviets with high grade military intelligence. During the autumn of 1942, "Lucy" provided the Soviets with detailed information about Case Blue, the German operations against Stalingrad and the Caucasus; during this period decisions taken in Berlin were arriving in Moscow on average within a ten-hour period; on one occasion in just six hours, not much longer than it took to reach German front line units. Roessler, and Rado's network, particularly Allan Foote, Rado's main radio operator, were prepared to work flat out to maintain the speed and flow of the information. At the peak of its operation, Rado's network was enciphering and sending several hundred messages per month, many of these from "Lucy". Meanwhile, Roessler alone had to do all the receiving, decoding and evaluating of the "Lucy" messages before passing them on; for him during this period it became a full-time operation. In the summer of 1943, the culmination of "Lucy's" success came in transmitting the details of Germany's plans for Operation Zitadelle, a planned summer offensive against the Kursk salient, which became a strategic defeat for the German army—the Battle of Kursk gave the Red Army the initiative on the eastern front for the remainder of the war.