Pride comes before the fall.
The simplest possible explanation for what happened today, 75 years ago.
The Axis had three major army groups in the Soviet Union.
Army group north which was besieging Leningrad.
Army group centre which still posed a potential threat to Moscow.
And Army group south.
Impatient and overly offensive thinking Hitler decided to split this last group in two. One part was to take the Kaukasus and conquer key oil fields. The other part was to reach the Wolga and capture Stalingrad.
This was a stupendous decision on many levels. First of all, too few troops were given too many objectives. And second of all, it made no sense to try and capture Stalingrad. According to some historians Hitler became obsessed with the city just because it was named after his opponent Stalin. What’s odd is that he did not try to capture Leningrad, but decided to starve the population to death in a protracted siege. In the case of Stalingrad he decided differently.
To cut the traffic on the Wolga made sense, but to engage in urban combat in Stalingrad, did not.
It denied the Germans their main advantage: mobility and superior tactics in tank warfare.
Before the attack in Stalingrad the Germans made the situation worse by bombing Stalingrad into one big pile of rubble. This turned it into an excellent defensive position… It was the largest bombardment on the Eastern Front. The bombardment formed a big black cloud of smoke in the form of a cross. German survivors would later say it was an omen. Stalingrad would become the grave of the German sixth army, arguably the best German army, which had conquered capital after capital.
The fighting in Stalingrad was sluggish. German soldiers were forced to conquer the city street by street, house by house. The cost was huge.
After intense combat and many casualties they were almost succesful. By november 1942 the Soviets were in control of only a small strip next to the Wolga.
Then the Soviets launched a massive counter-strike. On the 19th of November 1942 they started Operation Uranus to trap the Axis forces in Stalingrad.
While the Germans were fighting in the city, their flanks were protected by Romanian, Italian and Hungarian troops, who were ill-equipped and had low morale (in part because their officers treated the enlisted men like shit). They were badly mauled and their morale took a permanent beating.
These troops did their best to stall the Soviet horde, but they were no match for the well-prepared offensive. The Soviets broke through, and the two Soviet pincers united behind the Stalingrad front, effectively trapping about 290,000 Axis troops.
Hitler refused to let them break out. A break out only stood a chance if ordered immediately. Even then the troops would lose a lot of equipment, and would only be able to break out towards their comrades at a heavy price.
The Germans tried to keep the trapped men supplied by air. Even in the best of conditions this could not be done. Often they only got 20 percent of what they really needed to be effective.
A retreat would probably have doomed the group that was sent into the Kaukasus, as that group risked being cut off as well, if the troops at Stalingrad gave up. In a sense the Germans could choose: lose the troops in Stalingrad, or lose those in the Kaukasus.
The Sixth army and other units fought on until early February 1943. Their sacrifice was not completely senseless in military terms. The German public did not see it that way, and many felt betrayed. Even worse was that initially people were led to believe that there had been no survivors, that they all had fought to the last bullet. In reality 90,000 men were marched off into capitivity. Only about 6,000 would ever return to Germany, more than ten years later.
Hitler – the allies best asset in winning the war – is certainly to blame for this disaster. During the battle for Stalingrad, before the encirclement took place, he also ordered important armored units west, to stave off a potential invasion of France. This greatly hampered the tactical possibilities of the Axis around Stalingrad.
Pride comes before the fall. Hitler completely and systematically underestimated his enemies, and with lots of wishful thinking and a delusional belief in what will power can accomplish against impossible odds, tried to accomplish too much at the same time with too little forces.
The German army -on the tactical level- was so brilliant that it managed to recover and remain a vicious threat to the Soviet Union, well into 1943, and even until the early summer of 1944. Eventually one strategic blunder after the other destroyed most advantages the German army had over the Red Army. Berlin would fall two years and almost three months after the sixth army surrendered at Stalingrad.
More details about WWII, here: How Hitler could have won World War II