Hitler had already invaded Russia the summer of 1941, prior to declaring war on the US in December of that same year. He was never going to be equipped to fight a long war, especially through the harsh winters of Russia. The U.S. had already been out-producing Germany with weapons of war that were being sent to our allies before we even entered the war. Germany and Japan were never able to keep up with war production by the U.S. and Great Britain...and because Hitler was never able to get control of the necessary fuel supplies he needed for his war machines, his plans to conquer Russia were never going to come to fruition. Hitler didn’t count on the Russians being as tough as they were, and his troops weren’t equipped properly to fight in that kind of weather.
Had the Italians not been his allies he would have won.
The Italians invaded Greece without telling Hitler and they were getting their @$$es kicked by the Greeks, so Hitler had to detach a division and send it down to help the Italians subdue the Greeks. That division would have turned the tide at the battle of Stalingrad, and Hitler would have had his fuel. He would have taken Russia.