Oh gods. I'm sure we're all aware of just how involved any given battlefield really was during WW2, especially all the major ones have so much background information to them it is really next to impossible to give a one sentence solution like some kind of chess game, "Oh darn, if I didn't move that bishop there I wouldn't have lost." One of the real detractors from an objective view of the Stalingrad battle is just leaping into it cold, and regarding the period say late October 42 to February 43 when the story really begins outside Moscow.

Let's just have a look at vehicle serviceability by the Stalingrad approach for one. Not only had virtually the entire transport column been replaced by horses, including field weapon tractors (only sporadically complemented by rail), the XIV Panzer Corps had less than 200 mostly light Sköda tanks left by November, and infantry companies reported less than 80 men apiece (plenty operated at 60 men). Probably their only teeth was powerful support from Luftflotte IV. The invasion army hadn't been properly re-equipped since the previous year, and only patched and band-aided.

Not much of the 4th Panzerarmee actually made it to Stalingrad in the first place, suffering high attrition due to constant reassignments and very limited supply, arriving with 1 armoured and 1 motorised division and requiring immediate reinforcement from Paulus' meagre resources (by way of which he managed to perform the link up to the south...later which was to become the Uranus secondary target).

Hitler had taken increasingly direct command of all operations, had sent some Army officials to Dachau whilst others had promptly resigned. His plans were not even all that clear, he made clearly outlined demands of the Army, used fear and intimidation to ensure they were followed to the letter and then proceeded to contradict them himself, thus even when his instructions were followed to the letter one was still likely to be accused of treason. His field commanders noted (in diaries on the Caucasus), that finally all sense had departed the OKW and it was now dominated by unrealistic fanaticism and Hitler worship as it had never before been.

The satellite forces involved in supporting the Wehrmacht had originally relied upon Germany for modern weapons in the first place, making them more a drain than support. The Romanian anti-tank crews had for example only 37mm anti-tank guns, incapable of penetrating the armour of any major types then used (they could maybe knock out a T-70 scout tank if you're lucky). Their commanders consistently requested the Germans for 50mm guns still used in reserve by the Wehrmacht (having been replaced by the 75mm), but the situation on the Russian Front meant the Wehrmacht needed all the guns it could get for its own troops. Other anti-tank weapons produced by occupied territories like Czechoslovakia were needed for SPG production. As it was flak guns were being used more often than not in the anti-tank/artillery role by the Germans outside Stalingrad.

Then there was Paulus himself, not a babbling moron like Hitler and Göring but neither was he in any way brilliant or even distinguished. He was a Prussian conservative, an old school commander who believed in classical frontal warfare which the new Wehrmacht had been designed specifically to avoid at all costs. There's nothing particularly unusual about that in the time period, all the major field commanders aside from the "new generation" geniuses still believed in classical warfare. The English practised it, as did the Americans and Soviets, Italians and Japanese. Tactical operations were still somewhat a recently introduced oddity in the scope of strategic warfare, handled by all sorts of new and generally independent departments like SOE. But it did mean the 6th Army and Army Group B in general lacked some leadership qualities which were only belatedly recognised with the reasignment of Manstein (who as he described took command of a uselessly insignificant speck whose components were listed as expressions only).

But Manstein awaited proper re-equipment, which wouldn't happen until February as it turned out, with the assignment of recently reorganised and equipped Waffen-SS panzergrenadier regiments that were in fact, fully equipped heavy Panzer battalions complete with Tigers. That effectively turned the tide back to the side of the Germans with the Kharkov affair (which itself resulted of SS commanders directly disobeying Hitler's orders once in the field).

I mean there's just so much to Stalingrad, needless to say it was a foregone conclusion and there was little the Germans could have done at that time. Hitler needed to take the advice of OKH/OKW and dig in at rearward positions until the mid-43 re-equipment if he really wanted to win. Meanwhile form either one Front at Stalingrad, or one around Rostov (with expeditionary attacks into the Caucasus).

Hitler's impatience and foolhardiness with other peoples' lives cost everything.

That being said, the Soviets also displayed patience and foresight. Their soon to become infamous "concentric ring" defence system also worked beautifully to tie down diminishing Luftwaffe presence, which had been the primary strength Army Group B.