If you are into the intrigue & subterfuge of the war, check out "Bodyguard of Lies"
Everything we've seen has been part of a sequence of events in the war. BOB was invasion and post-invasion. Before the bombing campaigns went into full swing, there was the Battle of the Atlantic and the Kreigsmarine. The point was not just to establish shipping lanes, but the elimination of the U-boat threat. Enigma codebreakers and the US Ultra machines. The sacrifice of boats knowing they could be saved so as not to tip off the Germans that they were on to them. Insane choices that had to be made.
The prize of the campaigns was to neutralize the manufacturing capacity of Germany, but also the rail systems that would be used to support troops stationed in France to repel the invasions. That means bombing in France as well. That meant thousands of French citizens perished. As the neutralization of the U-boat threat was an equal prize in the Atlantic, so too was the elimination of the Luftwaffe in Europe. The German Air Intelligence Service (as seen in the episode) was vast and who's sole purpose was confounding Allied air attacks.
They could have stopped Neptune if even at a fraction of their capacity and capabilities. Had they had been able to conduct surveillance of Allied tactics, targets, and formations.....with their cryptoanalytics and wireless intelligence activities and their ability to blinding/jamming Allied radar and navigation systems....it could have been game over.
That's why the constant attacks had to be waged, knowing full well the losses that would come from them. That's why deception was the Allied game plan. Even with tens of thousands of planes attacking the Germans around the clock, they could have swept the Allies off the beaches had they known when/where.
One part I hope they touch on were all the German spies in the UK rounded up. With the codebreakers, they were able to find essentially all of them, turn them with the threat of death, and create their own army of double-agents feeding the Germans what they wanted them to know without tipping them off that their network had been compromised. That meant certain raids were flying into German defenses who knew they were coming....and they sent them anyway.
Anyway, long winded, but if you like this stuff, it's a MUST read.