Hitler´s attacked to the southern Soviet Union was an attempet to seize the oil fields in there. He thought that with it´s capture he would have enough fuel to face the rest of the war.
In order to did that Hitler deploys his army to the Causcasus area on 1942 to make an entrance for the southern Soviet Union and the oil fields in Maikop, Grozny and Baku. The Germany expectations of that invasion was that they would be quick and had no resistance from the Soviets but on the contrary the Soviet army fought back with ferocity making it hard for the Germans to gain terrain. The German army was so convinced that the capture of the oil fields were going to be easy that they hadn´t pack enough supplies to spend the rough winter in south of the Soviet Union and this along with the Soviets resistance caused the withdrawal of the forces and the non capture of the oil fields.
This was the beggining of the defeat of the Axis further in the war.
The operation made by the Red Army that compelled the German Army to withdraw was named "Operation Little Saturn" and was intended to cut them from the back and let them trapped as they had done it with "Operation Uranus" in the battle of Stalingrad.
I hope this answer help you. I give a little explanation of the motives and the battles held to add some context to the answer, hope you don´t mind. If you have any doubt about it will be a pleasure to answer it back. Regards.
