In 1942 fivethousand canadians landed at Dieppe in an attempt to capture the harbour. The attack ended in a bloody desaster. The supporting tanks got stuck on the beach and where unable to bypass the wall that was standing at the end of the beach. The tanks where sitting ducks for the German artillery. The canadians, supported by elements of the British marines and American Rangers of the 1st Battalion met heavy resistance. Only small elements where able to get off the beach. The absense of tank support once the men where of the beach wasn't the only problem. Communications with fighter aircraft flying over the beaches wasn't available and supporting fire from the fleat was much to light to make a difference against the heavy fortifications on the cliffs overlooking the beaches. The landing hadn't been proceeded by an air-stike by heavy bombers because Bomber Command held the opinion that it's day and night-attacks on Germany where so effective that it would eventually lead to the surrender of Germany. So it couldn't take bombers away from their attack on Germany to assist in other missions. This attitude of Bomber Command would later lead to conflicts between Eisenhowers staff and that of Bomber Command. In the Dieppe raid it ment that the soldiers that came out of the landing boats did not have any place to take cover and they had to make their way through a wood of obstacles. This demanded heavy casualties on the beach itself.
The allies lost 3500 men in the operation. 2469 men where injured and 900 killed.
The Allied headquarters learned from the raid on Dieppe that a direct attack on one of the harbours near the path of Calais would cost a very high toll while the chances of penetrating the defenses there where they where at their best where discutable.
COSSAC had made a plan to land in Normandy. This because a landing further north than the path of Calais would mean that it had to be done out of range of the fighterplanes at that time. Normandy and Brittany where in range of the fighter planes. Brittany was also discarded as an option because it could easily be locked off from the rest of France, which ment that it would be harder to break out of the beachhead.
The COSSAC plan for the invasion was an amphibious landing with three divisions on a narrow front. This area was what later would be known as Omaha Beach, Gold Beach and Juno Beach.
Montgomery changed this plan.
To give you an indication of the material that was needed for the invasion: On the first day the troops would need 900 tanks and armoured cars and 600 heavy guns. The three airborne divisions needed 867 gliders. the aircafts that woud drop the paratroopers and would tow the gliders would fly a number of sorties althrough the night and the following day.
Southern England was turning into one big storage in the months prior to the invasion. The air offensive that was needed to concer the air supremancy over Normandy demanded 163 new airbases in Southern England. 12400 hospital beds and 50.000 vehicles where ready and waiting. 300 kilometres extra rails where build to facilitate the transport of supplies.
The amphibious fleet that would assamble on d-day consisted of 5000 ships and 4000 smaller connecting boats. The fleet that would accompany and protect the landingships counted a total of 900 battle ships, cruisers, torpedo boats etc.