Could the Nazis truly have successfully invaded the Soviet Union?

As a big WW2-buff, I'm personally fascinated by this topic. When planning the invasion of Russia the Germans assumed a quick victory like over France, Poland or Greece. This plan was ruined just before Moscow around November-December 1941. Some say this was inevitable in the first place. Others say that the Germans came very close and that with a few better tactical decisions (like heading more directly for Moscow) and better planning (better Winter clothing - not to mention the Reich wasn't fully mobilized for war until early 1943 after Stalingrad) this could have succeeded. I strongly favor the latter view, as the Russians really were losing hopelessly before the winter paralyzed the Germans. After this point the war largely turned into a war of attrition. Again, some say this made the German's defeat inevitable, but this is also a questionable assumption. The Soviet Union's population in 1941 was just under 200 million, and this was with newly seized territories in the Baltic states, Finland, Poland and Romania. Before these, it was about 170 million. Germany was 80 million after seizing Austria, so the manpower ratio is about 2-2.5 to 1 depending on the figures used. Now things get complicated. Romania, Hungary and Finland were Germany's main allies on the Eastern front, and add about 30 million people between them. Italy also sent an army that eventually numbered about a quarter of a million. Italy's total population was an impressive 45 million, but only a fraction of their army was deployed on the Eastern front. If we accept a civilian/soldier ratio of about 10:1, we could add around 3 million to our population estimate.

Spain's fascist regime also sent a volunteer division in which about 30,000 served. Hundreds of thousands of other volunteers/conscripts from across Europe also joined the Wehrmacht or Waffen-SS to fight communism (for instance, about 50,000 from Holland, 20,000 from France, etc - a higher number than many care to admit). Bulgaria was also part of the Axis (with 7 million people) though it didn't sent troops to Russia.
The largest remaining group was the Russians themselves. It was estimated that around a million Russian defectors joined the Axis to fight Stalin's regime. Using the same 10:1 ratio, we'll add 10 million to the Axis population estimate and detract that from the Soviet figure (we'll use the higher one and say 200 million). Adding all this together, we get around 80+30+3+2+10 = 125 million Soviet Union = 200-10 = 190 million We end up with a ratio of about 3:2, hardly an insurmountable figure. The Axis figure could have been even higher if the Italians, Bulgarians or Spanish had contributed more. In fact, they would have slightly outnumbered the Soviets (42+7+25 = 74 + 125 = 199)

There are a number of other variables to alter this 3:2 figure. Significant Soviet forces remained in Siberia throughout the war, despite the deployment of a number of divisions west, to deter any Japanese aggression in the far east. Even larger German forces remained in France, Norway and elsewhere, and were gradually built up as the threat from the western Allies increased. Further questions need to be asked about the quality of the fighting forces. The German Allies (except perhaps the Finns) were no doubt inferior in quality to the Wehrmacht itself, but this was also usually true of the Red Army. Overall, the average Axis soldier was still likely of superior quality (in training, weapons, morale, etc) to the average Soviet.

The big question then is - what was the casualty ratio of the fighting armies as the war dragged on? The first six months of Barbarossa was a complete bloodbath for the Russians. Killed, missing, wounded and captured appears to have been upwards of five million. The Germans suffered 800,000 casualties. Even this doesn't tell the whole story, as only a quarter of the German casualties were killed, missing or captured - or about 200,000, while the same figure for the Russians was about 4 million. So that's a 20:1 ratio of soldiers effectively out of action, or at least 6:1 if we include wounded (presumably the real figure is somewhere in-between, we don't know exactly how many wounded soldiers returned to duty).

In short, the Germans went through the Russians like a hot knife through butter. If anything like this ratio had kept up for the entire war, the Soviets would have run out of men well before the Axis did, no problem. As the fighting dragged on however, the ratio gradually declined. Case Blue (the offensive towards Stalingrad in 1942) saw a ratio of just 2:1. Kursk in 1943 was actually higher, around 4:1, and Bagration in 1944 about 2:1. Even in 1945, when greatly outnumbered, the Germans generally inflicted higher casualties than they received. This also illustrates the vital importance of Stalingrad.

I found the Kursk figures surprising, as by 1943 the number of fighting troops was about 4 million to 7 million in favor of the Soviets, yet at a 4:1 casualty ratio, the Germans might yet have won. Stalingrad was perhaps different due to much of the fighting occurring in Winter, when we know the German's performance suffered considerably, and the fact that many Axis troops were encircled and then captured, which was still a rare phenomenon until the closing stages of the war. This shows that the significance of Stalingrad as the turning point of WW2 has hardly been exaggerated.
The key factors were the severity of Russia's terrain and climate, bitter tactical lessons learned by the Russians (don't get surrounded so easily), improved equipment (including lend-lease), better leadership and so on. These combined to slowly bring down the casualty ratio. Of course, the final factor I haven't mentioned was that before long Germany was fighting a war on two and then three fronts. If say a third of German strength was tied up in Italy and France by 1944, then we need to take 20-30 million off their population figure. This analysis just asks 'could the Germans have beaten the Russians' without taking into account the western allies yet. If Hitler hadn't declared war on the USA in December 1941, he might have been able to win a war of attrition anyway after Stalingrad. The British alone probably couldn't have won in North Africa or Italy, let alone invaded France.

The key fact is that, given a war of attrition and absent the western Allies, the Axis had to inflict a casualty ratio of at least 2:1 on the Soviets to keep winning the war. Once you take the western Allies into account, this number creeps up to about 3:1. After 1941, the Germans rarely were able to achieve this figure and thus lost the war.