https://en.wikipedia.org/wiki/John_Cairncross#Operation_Citadel https://en.wikipedia.org/wiki/John_Cairncross#Operation_Cita... Johny Carincross was a spy who

https://en.wikipedia.org/wiki/John_Cairncross#Operation_Citadel https://en.wikipedia.org/wiki/John_Cairncross#Operation_Cita... Johny Carincross was a spy who passed over raw intercepts decrypted with Tunny to the Soviet Union; so they got the German order of battle and other information in advance of the battle of Kursk; that sounds like a major contribution to winning the war (the spy was confirming information that the British passed on via official channels, however they seem to have put more trust in the reports of their spy). The outcome of the battle of Kursk did make a big difference. > "Intelligence never wins a war on its own," says Prof Ferris. Another event where spies/code breakers made a major difference is the battle of Moscow; https://en.wikipedia.org/wiki/Richard_Sorge#Wartime_intelligence https://en.wikipedia.org/wiki/Richard_Sorge#Wartime_intellig... "Various writers have speculated that the information allowed the release of Siberian divisions for the Battle of Moscow, where the German Army suffered its first strategic defeat in the war. To that end, Sorge's information might have been the most important military intelligence work in World War II. However, Sorge was not the only source of Soviet intelligence about Japan, as Soviet codebreakers had broken the Japanese diplomatic codes and so Moscow knew from signals intelligence that there would be no Japanese attack on the Soviet Union in 1941.[46] "