Russia’s defeat of France was the turning point because after this war, Napolean's own Empire went against his will of conquering entire Europe.
Explanation:
The setback that Napolean received in Russia opened the eyes of many. France had to accept a defeat in the battlefield and also suffered a huge loss of lives. With this war, the winning sequence of Naploean was stopped with a great disgrace to the French Empire.
The contemporary thinkers, nobles in the administration, and Napoleans closest advisors turned against him and started conceding him to stop the serial of wars which was undertaken with a view to conquer entire Europe.