D-Day was not decisive. It was the first step. The Allies didn't land to help the Soviet Union, they landed to keep
as much of Western Europe out of Soviet influence as possible.
What was decisive was the Reich's lack of oil reserves. They simply did not have enough oil to fight a war.
Lack of development of a heavy bomber, and long range fighter escort doomed them in the Battle of Britain.
And lack of reliable intelligence about Soviet manpower sealed their fate on the Russian front.
Even Hitler, when informed of red army numbers remarked, ‘If I knew they had this much, I wouldn't have invaded!'
So, lack of resources, lack of strategic planning, and a lack of intelligence pretty much made victory impossible.
Cool looking uniforms, and flags only get you so far.