When did German failure become virtually certain in WW2?

[Cassius]

There are probably three key junctures here:

June-December 1940: The success of the Dunkirk evacuation and the failure of the Battle of Britain (thus making it certain that Britain would continue to fight)  meant that Hitler lost his best opportunity to reorder the European state system along the lines that he wished.

June-December 1941: The failure of Barbarossa and the entry of the Americans into the war made it very unlikely that the Germans would ‘win’ the war.

October 1942-July 1943: The disaster in Stalingrad, the loss of the war in North Africa and subsequent invasion of Italy helped lock in not simply a German defeat, but the catastrophe of 1945.