As explained in Beevor’s book, Stalingrad, Stalin ordered Zhukov and Vasilevsky to come up with a plan and then agreed after some discussion with their plan. It was to keep just enough force in Stalingrad and fight just hard enough to keep the Germans fully committed to an urban war of attrition that would bleed the Germans and deplete their supplies. This was grind-it-out street by street, building by building, warfare, nothing like the blitzkreig operations the German units were built for. Meanwhile, the Soviets built up a huge extra army in secret. Their tanks and aircraft were being produced in Soviet factories that had been moved east of the Urals and significant supplies were being shipped in by the US and the UK . The huge numbers the Soviets were accumulating dwarfed German production and were far above what the Wehrmacht could actually deliver to its front line. When finally put into action, the armaments the Soviets had in late 1942 could hold their own against Panzers and Messerschmitts.
The Soviet plan , Operation Uranus, featured a two-pronged attack on exposed Axis flanks. It was timed for the onset of winter possibly in the hope the winter would incapacitate the German army as it had the previous year. It is hard to understand the relative lack of preparation the German High Command made for dealing with the 1942–43 Russian winter. It was the second winter in a row they were caught seemingly by surprise at the change of seasons. Maybe they were afraid to bring it up with the Fuhrer.
The Zhukov-Vasilevsky plan was aimed at completely encircling and defeating the German army and achieving a decisive victory. The Nazis fell into the trap. They were overconfident and suffered from a massive intelligence failure. They were blind-sided by the Soviet offensive coming from armies they did not even know existed. They fell into the assumption their poorly equipped Axis allies could hold the line and became more interested in blaming their partners than in making them more effective as fighting forces. The Soviets attacked the weak points, which the Germans felt was somehow unfair in war. The Axis command seem stunned by the Soviet attack which proceeded rapidly to achieve its objectives. Finally came the realization by Axis military leaders that they were being encircled and needed to retreat. But Hitler compounded the disaster by refusing to allow a retreat. Goering’s air supply effort would be woefully inadequate, as was obvious even before it began. The Soviets pushed westward making efforts to save Paulus more and more impractical, till surrender of his starving, frozen, and isolated troops was the only option. The Soviets achieved victory by assembling larger armies and using them more effectively. They had better leadership and a better strategy.