Robert Farley discusses the counterfactual in which Franco’s Spain joins the second world war and concludes that it wouldn’t have made much of a difference:
In Advanced Third Reich and its successor, World at War, Spanish entry can help put a nail in the coffin of a weak Britain in 1940 or 1941. Failing that it becomes a liability; Spanish control of Gibraltar tends to dissuade the Allies from wasting resources in the Mediterranean (this effect would likely have happened in the real world, as well), and the Axis simply lacks sufficient forces to defend the entire Atlantic seaboard from Allied amphibious assault. Since the game substantially understates Allied air and naval dominance from 1943–1945, I suspect that the overall net effect of Spanish entry would have been an extension of the war in Europe by a few months, and the end of Franco’s regime.
I can’t remember the details, but I once read an alternate history short story that cleverly details at some length the tale of Germany repulsing the D-Day landing and then skips ahead several years ending with the line “the sun rose early that morning over Dresden, brighter than a thousand suns.”
The point being that though in the real world, the development of nuclear weapons by the United States played no role in the more literarily interesting European theater of war, ultimately unless the Nazis had some plausible way to actually win the war before August 1945, Allies technological developments would have led to their defeat irrespective of the battlefield conditions in 1944–45. The real difference in these scenarios would have related to the post-war balance of power between the Soviet and American spheres of influence in Europe.