It was probably the most complex operation carried out in WWII. Operation Overlord (as it was called) had many features, some of which are listed below.
It involved many nations (Britain, The United States, Canada to name 3 of the major participants on the allied side.
It involved coordinating many types of armed forces. The infantry, the navy, the air force, paratroopers, spies and even civilians of one kind or another were involved.
It had a "hoax" component. That is what it was actually called. The Germans had to be persuaded that one beach was equally likely as another to be attacked. Also the deployment of men had to be very carefully disguised. There is one story that came out after the war, that tents had to be set up (with no one using them) to persuade the Germans that men would be deployed from this position (when in fact that was never the intent).
The date of the attack was a closely guarded military secret. Any time you go to that much care to guard a time and date, you know that the operation was quite complex.
Quite a few generals were involved. That alone would lead to its complexity.
These are just a few comments. D Day is an extremely interesting study, well worth your time.
Ethiopia was the only African nation that was not colonized, the Ethiopian forces successfully defeated the Italians during the battle of Adowa which was led by Menelik the second. This helped the nation kept their independent.Unlike other African nation Ethiopia had strong military force and huge sense of nationalism which made their resistance successful.
