After Citadel's abysmal failure, the Wehrmacht and Waffen SS had a massive hole in their defensive lines. The Soviets decided to take advantage of this window.
PIF reinforcements and the latest Pershing tanks on lend-lease came in to aid the Soviets. All in all, the Allies mustered over 2 million soldiers for this full scale assault.
The Germans on the other hand, were running out of petrol, and the Soviets and PIF commandos strategically destroyed their Nerve gas production facilities.