The Setting for the Landing...
Forces massed in England
More than 150,000 personnel, spread across southern Britain, arrayed for both tactical effectiveness and deception.
A crossing timed to the lunar cycle
Only two nights in the entire month of June were suitable, with the desired tidal conditions and lunar lighting
An attack in darkness, carried through night into morning
Airborne troops off course at night, missing or losing geographical reference points. Amphibious forces first landing just after midnight. It was to be a very, very long first day.
Vastly different levels of resistance, based on location
Because of the effectiveness of deception operations, beach defenses were neither uniform nor aligned well to the Allied attack. As a result, casualties in certain areas were dramatically higher than in others.
Americans to the West, British and Canadian forces to the East
Sword and Juno Beaches tasked to British forces, with Juno in the middle the responsibility of the Canadians. To the West, Utah and Omaha were the responsibility of the Americans.
Stout German fortifications - the "Atlantikwall"
German forces were filly dug in, with a vast network of concrete pillboxes, hardened artillery positions, ammunition storage capability, and fortified tunnels,