The massive allied invasion of Normandy in June 1944 was not necessary for the military defeat of Germany. The German Army had already been destroyed on the eastern front, and the German war industry was being devastated by the combined bombing offensive. According to Trumbull Higgins,
When the British were finally compelled by their Allies to invade France in 1944, it was an invasion essentially undertaken in the self-interest of the West, the terrible risk of the collapse of the Soviet Union having long since passed.
At this date the Red Army no longer needed more than Western supplies with which to occupy eastern Europe. (4:283)
The Normandy invasion was simply too late to be of meaningful assistance to the Russians. In fact, Stalin had conceded that is was no longer necessary.
Furthermore, many capable allied strategists knew that OVERLORD was no longer required and recommended against it.
Why were these recommendations not heeded, especially since they would have resulted in greatly reduced British and American casualties?
Two considerations cannot be ignored. First was the sheer momentum behind the OVERLORD planning. American planners had placed all their European "eggs" in this basket, they had been advocating OVERLORD against the British for over two years, and they were unwilling to concede to the British position in late 1943. Secondly, American leaders, including Roosevelt, felt that unless American forces took a significant (albeit late) share in defeating the German Army, the Russians would be entirely uncooperative in the post-war world and probably would
not assist in defeating the Japanese. The British were much less concerned about Russian sensitivities, feeling instead that their post-war interests would be better served by strengthening and conserving their armed forces rather than squandering them on the beaches of Normandy.
OVERLORD was not a military necessity; it was an unnecessary military gamble that could easily have failed. In retrospect, it is impossible to understand why American strategists were so committed to it. This commitment itself is evidence of serious strategic inflexibility. American planners either could not or would not adjust to the realities of the European theater in late 1943 and early 1944. Having already made the investment in a strategic bombing force that, in combination with the Russian Army, could have defeated Germany in a matter of months, why did the US not unleash the bombers and turn its attention to the Pacific theater? Why did US strategists not accept British recommendations for a less risky Mediterranean/Balkan strategy that would have left the western forces in a much more favorable post-war position relative to the Russians? The answers to these questions have political as well as military dimensions.
President Roosevelt believed he could buy Stalin's post-war cooperation. When Stalin expressed his final preference for OVERLORD at Tehran, he essentially allowed American political and military strategy to coalesce. OVERLORD was what the Russians still wanted and it was what Gen Marshall had always wanted. Roosevelt could not have been more pleased.