Effects of the successful defense of the Soviet Union against Germany on the opening of the western front in WW2

The Americans were fighting a two-front naval war, trying to stop the Japanese advance in the Pacific (which they did not succeed in doing until the Battle of Midway, in 1942) and clear the Atlantic of German surface raiders and U-boats in order to permit safer and larger-scale shipping of men and equipment.

Churchill had been long been pressing the Americans to to open a second front in Italy which he called the "Soft underbelly of Europe". The Americans did not wish a distraction from their intent to go through France, but Churchill was insistent. First, the Germans in North Africa proved tougher than expected but had to be defeated in order to move through the Mediterranean and the Suez Canal, and then the Italians fought harder than Churchill expected. The "soft underbelly" proved to be not so soft after all, and the American buildup for an invasion through France indeed went more slowly than the Allies had wanted.

In 1942, the western Allies were not nearly as concerned about an eventual Soviet victory as they were about an imminent Soviet collapse that would have left North America, South America and the British Isles alone against the Axis. They were too weak help the Soviet Union directly with an attack in western Europe, but what they did do was helpful all the same.

North Africa: This secured the Mediterranean and the Suez Canal for the Allies. Coming just before the Soviet counterattack at Stalingrad, it was a major diversion. The Axis lost a quarter million (mostly Italian) men, plus some German armor and half their air transport fleet in North Africa. Had these forces been later sent to Stalingrad instead, the Sixth army might have been maintained and relieved. If the Germans had won at Stalingrad in 1942, Eisenhower's victorious forces might have been sent to Iran, the Caucasus and/or the Volga to help "roll back" the German forces.

Italy: The Germans had to send some twenty divisions from the Eastern front to Italy after the battle of Kursk. This happened after they lost the battle, but even if they had won, the diversion of these divisions would have prevented them from "exploiting" their success.

So the Anglo-American plans were based on what they could do, not what they feared.

This interpretation of the history of WWII contradicts many well established facts. The most damaging one, in my opinion, is that Stalin actually pleaded for the opening of a significant European Western front in 1942/1943 in order to relieve the pressure on the Eastern front and was in fact disappointed in the Allies insistence that this could not happen before 1944. Also, I think the consensus view on diplomatic relations between the Allies is that Roosevelt was keen on building a strategic alliance with Stalin so that, far from trying to undercut him, he was rather favorably disposed towards an increase in the influence of the USSR (compared to the influence of the traditional European powers).

On the other hand, the rapid advance of Montgomery's 21st Army to Lübeck and Rostock in late April, 1945 can reasonably be described along the lines of the question: by that time (but not earlier), it was increasingly clear to Churchill in particular that Stalin's USSR would exert an unlimited influence on all territories behind the front line of the Red Army, so Montgomery's advance was made to preserve Denmark (and, presumably, Hamburg).

Re your claim:

I remember reading somewhere that the Allies (USA/UK) was not really keen on opening a second front to help the Soviet Union until the Soviets started to make progress in defeating Germany around 1942, 1943

Just the opposite was in fact true; the Western Allies were deathly afraid in 1942 that the Soviet Union might collapse (a la 1917) prior to their being able to open up a Second Front. Later, in 1943 and 1944, they were afraid that the Soviets might stop at the pre-war (ie 1940-1) borders and declare a truce with the Germans.

Both of these factors would stimulate the Western Allies to find ways to bleed off German resources in tertiary theatres, as they successfully did in Tunisia, Sicily, and Italy prior to the D-Day invasions of June 1944.

In The Last Lion - Defender of the Realm, Manchester and Reid note that at Casablanca Roosevelt said to Churchill that there would not be sufficient landing craft for a French invasion until Spring 1944. Admittedly this is partially a political decision, as once the Torch landings had been provisioned priority would shift to enabling offensive amphibious actions in the Pacific theatre for 12 months before shifting back to Europe. However even the immense economic engine of the U.S. had limitations, and took time to tool up and then produce the necessary machines of war.

However the additional time was probably required anyways. Early landings on the French North-West coast were problematic for the same reason that the Germans never seriously contemplated Sea Lion - amphibious landings in the face of determined opposition are difficult, and require extensive preparation. Without the Hobart Funnies that so eased landing on the British beaches Gold, Sword and Juno, and the Mulberry Harbours that enabled large-scale supply prior to capturing and clearing Cherbourg and Brest, the D-Day successes were far from guaranteed. All of these were developed in the 12 months leading up to June 1944.