Why was the German defeat at Stalingrad a turning point in the war

In World War II, Hitler split his forces in order to invade Russia.  The decision was a costly one and in essence, cost him the war.  To invade Russia meant fewer forces elsewhere, made Stalin and Russia an enemy, and put his soldiers' lives and equipment at risk.  With the invasion, supply lines grew very long, the soldiers were spread very thin, and Hitler did not anticipate the difficulties of fighting another front to the war.  The Soviet army and the people of the country dug in.  Winter, with its brutal cold, descended on the Germans. Tanks would no longer start and were useless with no fuel and the rough terrain, the Germans ran out of food and ammunition because of the long supply lines in the freezing cold, and many of Hitler's soldiers froze to death. The Russian invasion was a complete defeat for Hitler. 

Hitler's army never really recovered from the defeat at Stalingrad.  The aura of invincibility surrounding Hitler was now gone.  He could be defeated and the rest of the world now knew that they could win.