Was D-Day undertaken solely to beat Nazi Germany, or was there even in early 1944 a fear that the Soviet’s, left unchecked, would invade all of Western Europe too?

by SubmergedSublime

Listening to Ghosts of the Ostfront (Hardcore History) and it definitely gives the feeling that by the time of D-Day, Soviet dominance is already being established and that victory feels inevitable. With or without allied presence in the West. Did allied planners feel that way in 1944, or were we still convinced that a French invasion was necessary in order to ensure Nazi loss?

-Xotl

The question assumes that one can improvise the greatest amphibious invasion in history whenever it is expedient. In reality, such operations require months of planning and years of resource assembly.

Planning for what became Overlord began in 1942 (Operations Roundup and Sledgehammer, though the British were considering amphibious operations against the Continent even before the fall of France). But there were four major factors mitigating against a rapid launch of a full-scale invasion:

  1. Overly strong German garrison / reserve forces. Until the war could bleed away these reserves, this meant that any attack would face strong opposition. This dovetails nicely into...
  2. Weak attacking forces. The British had lost most of their equipment during the Fall of France. Operations in Greece and North Africa continued to bleed this out. Manpower was short too, compared to the Axis. The US entered the war in December 1941, but it would take time for American forces to be raised, equipped, shipped to Britain, and trained, especially since Japan also called on US resources, and Lend-Lease required the resources of both Britain and the US (albeit a small amount early on). Of course, part of what also made the attacking forces weak was a...
  3. Lack of transport craft. Perhaps hard to believe for a naval power like Britain, but the Allies were chronically short of the specialized small craft needed to move troops and vehicles to a hostile landing. Churchill would complain in his memoirs that it was incredible that something so seemingly simple could hold up things, but again and again operations were cut short or cancelled altogether (such as planned operations against Rhodes or Burma) due to this shortage. The bogging down of the Anzio operation (intended to be a coup de grâce) tied up ships that were intended to be used only temporarily, and the war against Japan (being almost entirely a naval campaign) demanded a steady number of landing ships as well.
  4. Lack of Allied air superiority. This wasn't achieved until 1944. Prior to this, the German ability to contest any invasion would have been much higher.

Then you need to consider the matter of weather. Launching a large-enough amphibious operation in the midst of winter weather was deemed impossible, so the invasion needed to be launched in the summer. As such, when it was postponed in 1942, it was assumed for 1943. When it was postponed in 1943, it was then scheduled for 1944. But the decision for 1944 was made in May 1943, at the Trident Conference in Washington (though the plan was not finalized at that time, the invasion decision was made). In May 1943 the Battle of Kursk hadn't even begun: the Germans were still deep in Soviet territory, and there was no assumption that the Soviets were about to rampage on their lonesome all throughout Europe and to the Channel.

When 1944 rolled around, the Germans were weaker, air superiority had been achieved, the U-boat threat was defeated, and the Allies felt themselves strong enough to launch. But the plans had already been made.