In March 1943, the German Wehrmacht achieved a victory against Soviet forces in Kharkov. While this did not end the Red Army, the soldiers of Stalin were not showing signs of being able to defeat the German invaders, even after the victory at Stalingrad.
On April 15th, Hitler issued orders to launch Operation Citadel to eliminate the Kursk salient, trapping many Soviet divisions and reopening the road to Moscow. The attack was meant to be a surprise, but the Soviets had been preparing an impressive defense in depth over 300km.
They knew the German plans. An Abwehr agent reported that since April 17th, Stalin had been fully aware of German intentions. Reinforcements flowed to the area from April 27th onward.
When the German offensive began on July 5th, success was impossible. It ended in defeat 6 days later, evaporating any chance of German victory in the East.
By this time, Admiral Canaris and Reinhard Gehlen had independently concluded the enemy was receiving rapid, detailed information on decisions by German High Command. Canaris believed the mole was Martin Bormann.