On D-Day-June 6, 1944-Allied armies landed in Normandy on
the northwestern coast of France, possibly the one most critical event
of World War II unfolded; for upon the outcome of the invasion hung
the fate of Europe. If the invasion failed, the United States might
turn its full attention to the enemy in the Pacific-Japan-leaving
Britain alone, with most of its resources spent in mounting the
invasion. That would enable Nazi Germany to muster all its strength
against the Soviet Union. By the time American forces returned to
Europe-if indeed, they ever returned-Germany might be master of the
entire continent.

Although fewer Allied ground troops went ashore on D-Day than
on the first day of the earlier invasion of Sicily, the invasion of
Normandy was in total history's greatest amphibious operation,
involving on the first day 5,000 ships, the largest armada ever
assembled; 11,000 aircraft (following months of preliminary
bombardment); and approximately 154,000 British, Canadian and
American soldiers, including 23,000 arriving by parachute and glider.
The invasion also involved a long-range deception plan on a scale the
world had never before seen and the clandestine operations of tens of
thousands of Allied resistance fighters in Nazi-occupied countries of
western Europe.

American General Dwight D. Eisenhower was named supreme
commander for the allies in Europe. British General, Sir Frederick
Morgan, established a combined American-British headquarters known as
COSSAC, for Chief of Staff to the Supreme Allied Commander. COSSAC
developed a number of plans for the Allies, most notable was that of
Operation Overlord, a full scale invasion of France across the English
Channel.

Eisenhower felt that COSSAC's plan was a sound operation.
After reviewing the disastrous hit-and-run raid in 1942 in Dieppe,
planners decided that the strength of German defenses required not a
number...