Historically, Germany was pathetically bad in the spy game. All their agents in Britain were caught and either turned or hung. The one attempt the Germans made to land agents in the US turned out as a tragi-comedy with all being caught within weeks of landing. But, what if the Germans had spy rings more like the Soviets seem to manage to get? That is, they have deep moles and effective spys that don't readily get caught. Here are some of the possibilities from this: The Germans know about the Normandy landings and other landings are are better prepared to meet them. This doesn't necessarily mean they change the outcome of the battle but, they do give the Allies far more casualties and a tougher fight. Spys in Britain and the US create sabotage of critical production in sophisticated and subtile ways. For example a mole might have contaminated basic materials for making vacuum tubes say at an RCA plant in the US. The tubes fail rapidly after being put in service making equipment they are used in highly unreliable. Or, a spy blows up one or another explosives plants in Britain or the US. Sabotage and far better intelligence might have been a great help to Germany. As it was Germany's spys contributed nothing to that nation's war efforts and may have materially hurt them in the case of turned agents and swallowing phoney planted information.