You should probably clarify your statement, as the U.S. HAD entered the war before the Battle of Stalingrad and even had operations in North Africa by that time and the invasion of Italy occured later the same year as the surrender of the Sixth Army at Stalingrad (1943).

You probably are meaning American involvement in D-Day but its important to remember this happened after North Africa and the invasion of Italy, both of which the U.S. was involved. Also, although the Germans had been retreating for almost a year when D-Day occurred, the creation and prior threat of a second front greatly inhibited Germany's ability to push the Russians back.

I'm not trying to discount the role and sacrifice the Russians played in toppling the Third Reich, just that they probably couldn't have done it without the U.S. and British.

As for when the war was clearly lost for Germany, I'd say the Battle of Kursk, but one could argue the Battle of the Bulge. I'm not qualified to answer whether the Germans had any chance of winning the Battle of the Bulge which is why I'd go with the Battle of Kursk, the last German counteroffensive in the Eastern front. Sidenote: the Battle of Kursk was a fairure due in part to a successful spy ring that delivered the overall plans to the Soviets in advance. Perhaps this could be cited as a critical point but that would be a huge stretch.

Germany had no real chance of winning the war by the time of the Bulge. To me the answer depends on some conditions. You could say it was when the Allies achieved superiority in material production, or you could make the argument that opening the second front (Russia) was the end of their chances. Certainly it could be argued that it was after the losses in Russia. And on and on.