I think alot of people focuses WAY to much on moscow and leningrad. This war was about resources.
spain could have been the best ally if they joined in 1940 and then helped germany close the med.
but in reality if turkey had joined in 1941, at the same time as rumania, it might have been game over for russia by the summer of 1942, They would have been able to effectivly defend iran from the soviet/british invasion of 41 too. It would probably be game over because of the oil in caucasus and the oil in iraq/iran. Germany lost mainly because of too little oil.
Once russia has fallen, along with egypt, germany could have sued for peace quite effectivly, offering to give back france, egypt and all of northern africa. If germany was lucky, they could have even broken the soviets in 1941 and might have been able to get their peace before US entered the war. Turkey would have gotten to keep iraq and syria, becoming the ottoman empire once more.