D-Day: The Invasion of Normandy When 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 happened; 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 attack. That would enable Nazi Germany to use all its strength against the Soviet Union. By the time American forces returned to Europe-if indeed, they ever returned-Germany might be in control of the entire continen