Axis air superiority early in the conflict limited the ability of the Soviets to mount such operations, whilst later in the conflict ongoing shortages of materiel, including silk for parachutes, was also a problem. Nonetheless, the Soviets maintained their doctrinal belief in the effectiveness of airborne forces, as part of their concept of "deep battle", throughout the war. The largest drop during the war was corp-sized (the Vyazma airborne Operation, the 4th Airborne Corps). It was unsuccessful. Airborne formations were used as elite infantry units however, and played a critical role in several battles. For example, at the Battle of Kursk, the Guards Airborne defended the eastern shoulder of the southern penetration and was critical to holding back the German penetration.