>>513869702
Nazi Germany's fighting strength on the Eastern Front was gradually drained throughout '42 and '43 by the actions of the western allies. The constant air battles across western europe, north africa and the mediterranean first crippled and the completely debilitated the Luftwaffe, which in turn granted the soviets air superiority in the east. Allied bombing campaigns throttled german logistics, exacerbated material shortages, and kept its economy contained and unable to expand to meet the needs of its widening fronts.
The direct military actions of the western allies, first in the Balkans delaying the start of Barbarossa (and thus ensuring the mud season would arrive before reaching Moscow); then in North Africa pulling a whole army group's worth of armour and equipment; and finally in Italy, forcing the cancellation of Citadel and drawing the largest and best-equipped german formations out of Kursk to enable a soviet counterattack, and then holding basically germany's entire strategic reserve (over 1 million troops with their most modern tanks and equipment) in a stalemate there for the remainder of the war.
The funniest part of the war is that Hitler kept roughly half of the the german army in reserve in the west throughout '42-'44, even as the eastern front was collapsing, convinced that an allied invasion of europe was imminent and that his one path to victory was defeating that invasion to bring the western allies to terms and then focusing everything east. And then that counterinvasion force was encircled and destroyed within months of the actual landings and its remnants driven back across the Rhine (only to be encircled and destroyed again in Ruhr and the Scheldt)