D-Day was successful because on the first day alone the Allies landed over 170,000 soldiers with almost 10,000 vehicles.
what nation that had signed a nonaggression pack with Germany eventually joined the allies
They, like their allies, the Germans, were successful until about 1943. They both were clearly losing the war by then.
yes
Axis verses Allies.
The soviet Union
because the Germans thought the Allies were landing somewhere else
Germany was hated after WWI. By and large they had no allies. This, by the way, was in large part what led eventually to WWII. There is a lesson here.
The Allies eventually gained control of all the oceans. That was a big part of winning WW2.
The Allies defeated Axis forces in North Africa, which set up the successful Allied invasion of Italy.
There was no response, not even diplomatic or economic sanctions. This led Hitler to be more and more draconian with the Jews and eventually led to the Holocaust.
They learned that air superiority is essential before a land invasion can be successful.