Together with the Soviets. When D-Day took place Germany was already losing to Soviet Russia.
If Germans were as numerous as Americans or Russians they would win WWII.
The Allies were actually outnumbered by Axis troops in the European theater.
When D-Day took place the Soviets were not invading Germany yet. They were waiting to see if D-Day was a success.
The Nazis had lost their battle to conquer the Russian heartland, but they weren't yet defending their border.
The Nazis lost to the US and UK in Europe despite the fact that they had the advantage of defensive posture, interior lines of communication and supply and the advantage of numbers.
In fact, in the most famous battle of the European theatre, the Battle of the Bulge, the Nazis were able to attack and pin down 80,000 Allied troops with a force of more than 200,000 men.
But the US won that battle, despite being vastly outnumbered and surrounded.