The turning point is when the Germans became bogged down in late 41. This bought the Soviets the time they desperately needed to recover and reorganize. Production of essential arms and equipment was increased. T-34s and Il-2s were rolling off the production lines in far greater numbers. This increase in production coupled with the equipment provided by the Lend-Lease Program was essential to the ultimate victory by the Soviets. Stalin also reversed his policy of purging the officer corps. This policy had left Soviet leadership in shambles. Tactics were also changed from WWI-like tactics to a form of their own blitzkrieg with high mobility, air supremacy, and armor-infantry coordination.