One of the reasons, if no the main reason, that (RAF) Bomber Command reduced operations into Germany in 1944 was due to the requirements for the run up to D-Day... It had little to do with the losses incurred by the Luftwaffe night fighter arm or flak.
That's not what the RAF's own official history says.
It's exactly what the official history says.
Early in January, 1944, Harris made known his views in the clearest possible manner. 'The only efficient support which Bomber Command can give to "OVERLORD"', he said, 'is the intensification of attacks on suitable industrial centres in Germany as and when opportunity offers. If,' he went on, 'we attempt to substitute for this process attacks on gun emplacements, beach defences, communications or dumps in occupied territory, we shall commit the irremediable error of diverting our best weapon from the military function for which it has been equipped and trained to tasks which it cannot effectively carry out. Though this might give a specious appearance of supporting the army, in reality it would be the greatest disservice we could do them. It would lead directly to disaster.'
By adopting Zuckerman's conclusions, of which the most important was that the best method of destroying railways was to attack maintenance and repair facilities, Leigh-Mallory showed his hand and at once aroused strong opposition. Such attacks, said the critics, would mean heavy casualties among French civilians at a moment when their goodwill would be more than ever needed. Even members of the Air Staff, whose professional instincts urged them to support the attack on railways, were influenced by this political consideration. So also were other Service and civilian chiefs, and even the Supreme Commander himself, while Spaatz and Harris continued vigorously to protest against what they felt to be a grave misuse of their forces.
Leigh-Mallory, however, persevered, being supported steadily by Tedder. Eisenhower's deputy had had experience in Italy of the bombing of railways and the means of transportation and was convinced that such a policy would achieve great, perhaps decisive, results in France. By the beginning of March Leigh-Mallory's Committee had drawn up a list of seventy-five railway targets comprising the major servicing and repair centres in northern France and Belgium. He at once pressed for permission to attack them. To damage or destroy them, he said, would compel the enemy to move from the railways to the roads and the delay thus imposed would assuredly be fatal. It was at length decided to make trial of this plan, and on the night of 6th/7th March, 1944, 263 aircraft of Bomber Command dropped 1,258 tons of bombs on the railway centre at Trappes, south-west of Paris. The results were striking. Tracks, engine sheds and rolling stock were so heavily damaged that the centre was out of action for more than a month. Eight further attacks by Bomber Command, in strength varying from 300 to 80 aircraft, were made during the month on other rail centres. While the full measure of the success they achieved was not known at that time, sufficient evidence was soon available to show the soundness of a plan unswervingly urged by Leigh-Mallory from the beginning, and to which Eisenhower had in due course become converted. At the end of March, at a meeting convened by the Supreme Commander and attended by the heads of the two air forces and by the Chief of the Air Staff, it was decided that the 'Transportation Plan', as it was then known, would, despite the possible odium it might arouse, offer the best chance of success. A week later the matter was discussed by the Defence Committee of the Cabinet, which was attended by the heads of the three Services. All urged the adoption of the plan, and after much debate the inevitable compromise--in this instance to be disregarded almost before it was reached--was adopted. It was decided that attacks on railways must be restricted to places where the risk of causing casualties among the civilian population would be comparatively small. Eventually it was suggested that the list of targets should be revised and only centres where casualties among the French were unlikely to exceed 150 should be bombed. This restriction was even included in the final directive. It was very soon ascertained that, in fact, the casualties, though sometimes grievous, were not nearly so heavy as had been feared, and that on some occasions the number of Germans killed had exceeded the number of French. On 15th April, Tedder issued a complete list of 'Transportation' targets to the United States Air Forces and to Bomber Command, and informed Spaatz and Harris that the 'Transportation Plan' had been approved.
Harris wanted to continue bombing Germany, Tedder, backed by Eisenhower, wanted to use the strategic bombers to destroy transport targets, mostly in France, in preparation for the invasion.
And it wasn't just invasion preparation. A lot of sorties were also flown against V-1 sites at the same time.
