Operation "Barbarossa" the German plan to invade the Soviet Union was a colossal success until Stalingrad. Capturing one third of the European Soviet Union and having the Luftwaffe taking monopoly of the sky, encircling the Russians in the Ukraine all German success was neutralized in the Soviet winter offensive. Even more it turned against them. From where such radical turn point of the situation? Is it a result giving allied aid to ports such as Murmansk?
First, Barbarossa was not a unmitigated success; while the territorial gains and Soviet losses were impressive:
The German plan counted on the Soviet Army collapsing in the first month of war, because they did not have logistics to keep fighting so far from their bases. In fact, in december 1941 the Russians were able to push the Germans back in Moscow.
- By Spring 1942 the Red Army was considerably reinforced from the losses, but Stalin kept most of it around Moscow, so the push for Stalingrad was relatively easy for the Germans.
Most of the industry was successfully moved out of reach of the Germans.
Then, at the start of the Russian troops and officers had very little combat experience at the beginning, compared with the Germans who had been at war for two years. During one year, the Russians had been raise and equipe new armies to fight those that the Germans already had standing, it was only slowly that they achieved some degree of parity.
Stalin interference with the military (including the political comissar system) hampered the fighting ability of the Red Army and allowed some more catastrophical defeats, and he eventually began to learn that lesson. And, in opposite, the fast advance in 1941 and the defeat of Russian offensives in 1942 (often caused by Stalin asking unrealistic objectives) led Hitler to confirm his theories 1 regarding the Russians as sub-human. That combined with Germany dire need of the Caucasus resources, and made believe Hitler that the German army (and his allies) could maintain a front so long that it could not be properly defended2.
Also, while a heavy defeat for the Germans, Stalingrad did not really "turn the tide" of war. It just showed that Germany no longer had the absolute superiority it had enjoyed against Poland, Norway, Denmark, Belgium, Netherlands, France, Yugoslavia and Greece. The German Army remained an operational force, higly capable of defending their lines and launch offensives (just after Stalingrad, Russians overextended themselves again and were severely beaten at Jarkov). The change of the strategic situation was more marked after the battles of Kursk and Bagration.
All of the above does not mean at all that Western supply was not important 3; it was an important factor in the Soviet victories. What I want to point is that the victory at Stalingrad, while manifested itself suddenly, was not based in a simple variable (foreign aid yes/no) but in a series of processes that took their time (and remember that you would have to wait one year more before getting to the real turning point of the Eastern Front).
For references, apart from the Wikipedia articles about Barbarossa, Battle of Moscow, Stalingrad and Kursk, you can look out in youtube the Battlefied series episode about Stalingrad (I doubt that the poster has permissions to distribute it so I will not provide a link, but it should be easy to find).
1 In the Series "Battlefied", in the chapter about Stalingrad, it is stated that while the German advance from the Don to the Volga was a stream of Soviet defeats, then the Russian troops were able to retire in good order. Hitler believed that the absence of giant encirclements of Russian troops meant that the Red Army was near its collapse, when it just meant that it was fighting better than previously.
2 The Romanian troops that were the first attacked by the Soviets knew of the preparations of the Soviets and that they were not prepared to properly defend their sectors, but their pleas for reinforcements were dismissed. Probably the Germans thought they could stop (as they had before) any Russian attack.
3 I remember a citation Zukov, the Soviet general in charge of the encirclement, said the most decissive factor was "Spam", canned food provided by the USA and that allowed keeping them supplied. Unfortunately, I cannot find a reference.
Stalingrad was less of a "radical turn point" as it was an inevitable consequence of a long war between the Germans and the Soviets. Similar to Japan, Germany was not prepared for a long war, while the Soviets were not prepared for a short one. There had been plenty of other times when the Germans were encircled on the Eastern Front, but they always managed to hold out, break out, and turn it into a victory. But at Stalingrad, Germany had run its armies ragged, and the Soviets had adapted sufficiently to German tactics.
The Germany army of 1939 to 1941 was something of a bluff. It was still primarily horse drawn. Most of its tanks were woefully undergunned and underarmored, the Panzer I and Panzer II. Their infantry mostly used bolt-action rifles of WWI vintage. What it did have going for it was an incredible ability to react to changing situations at a pace no other army could.
They used this defensively, responding to enemy movements and attacks, and offensively by making sudden, concentrated attacks where the enemy did not expect them. By the time the enemy reacted, the situation had already changed. The key elements of early German victory were the radio, unlike other armies every tank had one, tactical air support, massed armor, and an amazing general staff. Their forces could bring down a lot of concentrated hurt faster than the enemy could react.
Germany won the Battle Of France, in part, because they outmaneuvered the Allies, but also because it was over so fast. The Germans and the Allies fought the Battle of France with what they had available in those six weeks, that means not just material, but also tactics. The Allies could not match the new German pace of war, and the French lost half their army and surrendered the other half.
Germany hoped to do the same by attacking the Soviet Union, and they nearly did, but the scale was so much larger than France both in terms of sheer size of territory to be conquered, the people to be conquered, and the size of the Soviet Army (which the Germans underestimated). Though they gained local superiority in the initial attack, a German specialty, overall they were fighting an army twice their size. And they'd have to do it on a 3000 km continuous front, while also fighting in North Africa, garrisoning Norway, France, the Balkans, and fighting the Battle of the Atlantic. To say they stretched themselves a bit thin is an understatement.
The more they fought, the weaker they got.
The Soviets, in contrast, had the kernel of a modern army inside an inept one. The Soviet army of the early 1930s was one of the most innovative and potentially powerful in Europe. They'd heavily invested in tanks, air power, infantry firepower, and artillery, plus new tactics to use these new military inventions. They had their own version of Blitzkrieg, Deep Battle, concentrating mobile armored forces into a concentrated fist to break the lines and run wild in the rear.
Then the political purges came, the upper ranks of the military were replaced with inept political stooges. The army was remodeled on communist lines with the infantry being the key weapon and the tanks were dispersed to support them. But that kernel remained in tanks like the KV-1 and T-34, and commanders like Georgy Zhukov who survived the purges by keeping his head down in an obscure far eastern post.
The Soviet humiliation in the Winter War with Finland turned their army into the laughing stock of Europe, and some of the ravages of the purges began to be reversed. The German invasion greatly accelerated this process as harsh reality overrode political dogma.
Like the French, the Soviets also lost half their army. Unlike the French, it took five months, not six weeks, to do it. And unlike the French, they had another army equal to the Germans ready to replace it. This army had a year to learn. And this army had some excellent weapons to rearm itself with, all those existing weapons like the KV-1 and T-34 that were only just rolling off the production lines when the Germans invaded.
And the Soviets were learning, at great cost, but they had the time to learn this new pace of warfare the Germans had brought with them. They learned how the Germans operated, their strengths, and their weaknesses, and they learned how to counter them.
The more they fought, the stronger they got.
By the summer of 1942, the German army had run itself ragged. It was now fighting three major campaigns in the East alone. In the North to take Leningrad, still holding out, in the Center to take Moscow, both a political and transport hub, and in the South against the Caucus oil fields.
Soviet roads and rail lines were no match for modern French and German highways, transport was slow. It didn't help that Hitler started meddling. With his poor understanding of logistics, he shuffled forces around that looked easy enough on paper, but caused massive traffic jams, confusion, and delays at the front. One of those was shuffling and reshuffling his armored columns tearing across the southern Soviet Union between the targets of the Caucuses and Stalingrad causing delays and confusion.
While the siege of the city gets all the attention, the real Battle of Stalingrad was won and lost at the Don River.
The German push to the south was creating a larger and larger strategic salient. Their flank was anchored on the Don River to the north, but the German army was stretched so thin they increasingly had to rely on allied units from Italy, Romania, and Hungary to guard their flank. Poorly equipped and their hearts not really in the fight, they would be the German's real downfall.
The German army could no longer cover the enormous front it had created. The Germans again underestimated the Soviets and did not think it possible they had any reserve to mount a counter-offensive, and again they were wrong.
A million men, 1000 tanks, and 13,000 artillery pieces crashed into the weak covering forces north and south of Stalingrad. They blew through the Romans and Italians and surrounded the German armored spearhead of just 300,000. The quick operational thinking of the German staff could have saved the entrapped army, but a lack of fuel, and Hitler's meddling meant they remained obsessed with Stalingrad and holding out. By the time a breakout was attempted, the entrapped army was too weak to mount one, and the Soviet encirclement was too strong to be broken from the hastily thrown together relief columns from outside.
This is a long-winded way to say that the German logistical situation was such that if not Stalingrad, it would have been somewhere else.
In 1942 the Soviets could still afford to take large losses, while the Germans could barely hold onto what they had. The Soviets could hold back a million men for a counter-offensive, while the Germans had to use unreliable satellite armies.
While losing Stalingrad would have been a blow to the Soviets, it would not have ended the war. The Germans would be past the Volga River with no natural barriers to stop them pushing further east, but where would they push to? The oil was to the south. Moscow was to the north. The Germans no longer had the forces to exploit such a strategic breakthrough.
Instead, the Germans would have further stretched their lines, their logistics would have gotten even worse, their spearheads even more ragged, and the inevitable Soviet counter-offensive would have fallen on some other poorly defended flank.
Germany lost an army of 200.000 to 300.000 in Stalingrad, 6th Army, part of "Heeresgruppe Süd". The German field army had 4.000.000 personnel in 1942 [Creveld, Fighting Power, German and US Army Performance, 1939-1945], which made this a loss of manpower of 5%.
The German army was pushed back suffering very heavy losses. This was the first major defeat for the German war machine and starting from this point onwards it was the Soviets who seemed "undefeatable".
The German expansion was at it's maximum in late 1942 and after the battle of Stalingrad the German expansion shrunk more and more. So Stalingrad was the turning point - but bot the decision point.