The German offensive to take Stalingrad, the battle inside the city, and the Soviet counter-offensive which eventually trapped and destroyed the German 6th Army and other Axis forces around the city, was the first large-scale German land defeat of World War II
It's important to note that Hitler's directive did not demand to occupy the city of Stalingrad. The directive was "to reach Stalingrad itself, or at least to cover it with heavy artillery, so that it will no longer be an industrial or transportation center"
It was the stubborn battle to occupy the city itself to the last ruined meter, and later Hitler's refusal to retreat from Stalingrad, that cost him his entire southern campaign, and horrible losses to both sides
In April 1942, Hitler issued "war directive 41", which detailed his plan for the Russian front for summer 1942, code named Operation Blue.