Battle of Kursk
History - Military history -- List of battles -- World War II
The Battle of Kursk was a decisive battle of World War II. It was the largest armoured clash, and included the most costly single day of aerial warfare, in history.
The Germans needed to regain the initiative after the German Sixth Army surrendered at Stalingrad in January 1943.
Hitler chose the Kursk salient into the heart of the Ukraine, and on July 5, 1943, launched Operation Citadel, which proved to be their last offensive on the Eastern Front. The Soviets, alerted by the "Lucy" spy network in Switzerland, was prepared to defend in depth.
Obviously, much more needs to be written.
Wikipedia contains spoilers: the Germans lose.