Many people, impressed by the casualties inflicted by the Red Army on the Wehrmacht and the overwhelming military strength of the Red Army in the closing days of the war viz a viz the Nazis, seem to believe that the Red Army "did all the work" in defeating Nazi Germany and that the Western Allies were superfluous.
Though reasonable, this is wrong. Even prior to the appropriation of French, low country, and Scandinavian resources by the Nazis, the relative war potential of Germany was greater than the USSR and its share of world manufacturing output even slightly lower. If you include German allies, like Japan and the Slavic nations , it climbs even higher.
Further, despite the miraculous efforts of Soviet engineers in saving a large portion of their productive output in the summer of 1941, a large minority of Russian manufacturing was lost in the first few months of the Nazi invasion.
Given the great operational superiority of the Wehrmacht (though that gap was narrowed during the course of the war), this USSR resource inferiority would likely lead to a quick defeat at worst or a stablemate in western Russia at best.
So, why didn't that happen? The Western Allies contributed to victory in several ways.
1. Strategic distraction of Japan:
While the USSR was able to fend of the initial assault by the Nazis, it did so by transferring the battle hardened crack divisions from Siberia to defend Moscow. If the Japanese had attacked Siberia simultaneously with the Nazis, it is unlikely that the USSR would have survived as any kind of viable state.
Why didn't the Japanese do this? Partially, it was due to the defeats inflicted by the Zhukov in previous border wars. But the main reason is that American intransigence over Indochina and China had lead to a prior strategic commitment to defeat the United States. By the time the Nazis attacked, the Japanese were strategically committed to a conflict with the United States. And this was certainly true by the time the Siberian divisions had been removed,
2. Lend-lease supplies to the USSR through Tehran and Arkangel.
These were extraordinarily significant since they made up the manufacturing shortfall between the USSR and the Nazis. Further, they allowed Russian manufacturing (relocated to beyond the Urals after the summer of 1941) to concentrate on the production of simply made but effective weapons: the T-34 tank, the IL-2 attack craft, and the Yak-9 fighter. These supplies included:
- Tens of thousands of locomotives
- Hundreds of thousands of jeeps and trucks
- Millions of pairs of boots
- Thousands of combat aircraft and tanks
Without these supplies, the USSR would have starved AND long-range Russian offensives would have been logistically impossible. Operation Bagration and Uranus (the operations that destroyed Army Group center and the forces around Stalingrad respectively) simply wouldn't have happened.
3. The Western Allied Strategic Bombing Campaign
The strategic bombing campaign constituted a second front in and of itself. And this is true in two ways
A. Resource distraction
The bombing campaign distracted Nazi resources away from the Russian front. There are many ways this is true but three are most important. First, hundreds of thousands of troops were needed to man airfields, radar stations, anti-aircraft regiments, and civil defense battalions.
Second, the Luftwaffe was perpetually under-manned on the Russian front, despite the fact that control of the air was vital to Nazi operational control of the initiative. Why were they so short? Because a)the bombing campaign forced most of the fighter strength to be focused in Germany itself for defense and b) the necessity of defending Germany meant that bomber production essentially ceased in 1942 because more fighters were constantly needed.
Finally, and perhaps most importantly, the need for antiaircraft protection of the Ruhr and other German cities meant that 75 PERCENT OF ALL 88MM AA/AT guns were in Germany. It should be pointed out that the German 88mm gun was the finest piece of artillery in any army at the time. It was also the only effective means for destroying heavy Soviet KV-2 tanks. Nazi armies would certainly have been more effective if more of hose guns had been made available to them in Russia.
B. Decreasing Nazi manufacturing capacity.
After Albert Speer took over Nazi industrial planning, Nazi production sky-rocketed. In fact, in 1944, Nazi Germany produced more fighters than the USSR and more tanks than in any other previous year (and better tanks too).
However, the standard is not "did Nazi Germany produce more each year?" but "how much would they have produced had there been no bombing?" By forcing Speer to decentralize production and destroying manufacturing capacity, the bombing campaign had a significantly retarding effect on German industrial production, Further, once the bombing campaign focused on oil production and refinement, the German economy was brought to an utter standstill by the end of 1944.
We can safely conclude that without the Allied bombing campaign, the Nazis would have been able to produce more and made more available to the Russian front. It is not unreasonable to think this would have made a decisive difference in either winning or stalemating the Eastern front.
3) Distraction and destruction of German combat power
While the majority of German casualties were suffered on the Russian front, the Western Allies did contribute non-trivially to the destruction of German ground forces. Further, it should be pointed out that the existence of the Western Allies contributed mightily to a strategic dispersion of German military resources to respond to even a threat of an invasion.
A) North Africa
After Operation Torch and Hitler wisely decided to reinforce Rommel's forces despite the fact that it had no real chance of resisting the combined might of Montgomery and Eisenhower (particularly after Patton took control). This lead to a destruction of some 15 German divisions and casualties roughly equivalent to the final defeat at Stalingrad. In fact, German generals called it "Tunisgrad" behind Hitler's back.
It should be noted that almost every division reconstituted after Stalingrad was sent to reinforce the Western front and not to the Eastern. After the Battle of Kursk, the rate of reinforcement for the Eastern armies dropped off dramatically and largely explains the awesome operational success of the Red Army.
B) Italy and Yugoslavia
The threat of allied invasion of Yugoslavia or Italy distracted many German divisions (Germans were forced to occupy Yugoslavia and Greece with as many as 30 divisions at one point). The invasion of Italy was not particularly efficient because Kesserling's defensive genius (along with generally unimaginative Allied tactics) meant that the Allies had to contribute more divisions than they distracted, but they did distract over 15 divisions and inflicted significant casualties at times.
The threat of a cross-channel invasion of France is one that rightly concerned Hitler. On June 6, 1944, Hitler had 60 full strength divisions in France and another 20 in Scandinavia to guard against a Western invasion of Europe. Most importantly, those 60 divisions contained most of the best equipped and trained divisions in the Wehrmacht (Panzer Lehr, SS panzer divisions of various types) .
The fighting before Caen and in the hedgerows of Normandy wore down the best divisions in the German Army, and the fighting after Mortain (including the famous Falaise pocket) destroyed a large portion of Hitler's armored reserve.
D) The Battle of the Bulge
The Western Allies destroyed Hitler's last strategic reserve which could have significantly held up Soviet advances and might have stalemated the Eastern Front given the logistical and operational difficulties that faced the Red Army at the time (see the fighting in Mayre's Heights before the fall of Berlin).
This is not to belittle the achievement of the Red Army in WWII. Despite devastating losses in both their operational strength and sustaining industry, the Red Army was able to bounce back and largely close the operational gap between itself and the Wehrmacht. In doing so, it destroyed hundreds of German divisions and inflicted millions of casualties,
However, Western Allied involvement was neither insignificant nor superfluous. Without such contributions, the USSR wouldn't have come close to victory, much less the overwhelming victory that eventually ensued. In all likelihood, the USSR would have lost and lost badly.