Although it is a general military term, D-day has become synonymous with the Allied invasion of Normandy, France—code-named "Operation Overlord"— on June 6, 1944, during World War II. Following the German invasion of the Soviet Union in 1941, Soviet premier Joseph Stalin called on the Allies to open a second front in western Europe.

By May 1943 such a plan had become the Allies' number one priority. At a meeting held in Quebec, Canada, Lieutenant General Frederick Morgan, chief of staff to the Supreme Allied Command, presented a preliminary plan to the Allied leadership.

With input from Lord Louis Mountbatten, chief of the British War Department's Combined Operations Division, Field Marshal Bernard L. Montgomery, and numerous others the invasion plan began to take shape; by D-day close to 30,000 civilian and military personnel had worked on the plan in some capacity.

Officially, "Overlord" was the overall designation for the Allied offensive that would run from June to August 1944; the naval and beach assault operations on the day of June 6 were code-named "Operation Neptune," with various related operations, such as airborne drops, given their own code names.

To gain a foothold on mainland Europe and liberate it from Nazi occupation, "Neptune" involved a strategy of coordinated attack from the air, sea, and land that culminated in an amphibious assault by Allied forces—composed of U.S., British, and Canadian troops—upon the German-held beaches of Normandy in northern France.

In December 1943 American general Dwight D. Eisenhower was chosen as supreme Allied commander, with three British commanders in charge, respectively, of air, sea, and land forces: Air Chief Marshal Sir Trafford LeighMallory, Admiral Sir Bertram H. Ramsay, and Field Marshal Montgomery.

D-Day naval bombardments map

Likewise, Air Chief Marshal Sir Arthur Tedder, the deputy supreme Allied commander, and General Walter Bedell Smith, Eisenhower's chief of staff, supervised the massive logistical task of coordinating the men and materials needed for the invasion.

Before settling on Normandy, Allied commanders had considered the Pas de Calais, the narrowest point in the English Channel between England and France. However, Mountbatten felt that although Normandy was farther away, it offered an ideal location for two main reasons: long, sheltered beaches that would be less defensible, theoretically, than Calais and two large ports vital to the invasion fleet, Cherbourg and Le Havre, which could be captured by land.

As commander of all ground forces, Montgomery pushed for five beachheads, which Eisenhower endorsed—"Utah" and "Omaha," assigned to the American-led Western Task Force, and "Gold," "Juno," and "Sword," assigned to the Britishled Eastern Task Force.

army landing

Both task forces comprised the 21st Army Group, consisting of the British Second Army, commanded by Lieutenant General Sir Miles Dempsey; the Canadian First Army, commanded by General Henry D. G. Crerar; and the U.S. First Army, commanded by Lieutenant General Omar N. Bradley.

For the Germans, Field Marshal Gerd von Rundstedt commanded all forces in western Europe (Oberbefehlshaber West), consisting of Army Groups (Heeresgruppen) B and G; Field Marshal Erwin Rommel commanded Group B, which was given the task of defending the channel coast. Because of the fight with the Soviet Union that reduced troop strength in the west, Adolf Hitler charged Rommel with shoring up gaps in the coastal defenses that exposed Germany's western flank to invasion.

Coined the "Atlantic Wall"—consisting of concrete bunkers, gun emplacements, and varied obstacles on land and in the sea that extended along the English Channel, the Atlantic, and the French Mediterranean—by May 1944 the Germans had poured close to 18 million cubic meters of concrete and placed over half a million obstacles. Rundstedt and Rommel disagreed, however, on how to defend against an Allied threat.

Rundstedt pushed for a central reserve farther inland that could counterattack once Allied intentions were known; Rommel, on the other hand, advocated confrontation at the point of invasion, with the strongest units readied to "push them back into the sea." With neither willing to concede, a plan developed that encompassed both ideas—which would prove ineffective in the end.

The Calais Deception

Despite the Allies' choice of Normandy, Calais still played an integral part in their plan. Many in the German High Command, most notably Hitler himself, believed Calais to be the genuine target of any Allied offensive against the mainland.

Through a deception operation known as Operation Fortitude, the Allies broadcast fake radio traffic and invented nonexistent armies that pointed toward an invasion at Calais. Hitler and the High Command, headed by Field Marshal Wilhelm Keitel, believed that any actions by the Allies against the mainland would simply be a diversionary tactic to draw away from the real target of Calais.

Consequently, the Germans concentrated a majority of their best reserves, including the powerful 15th Army (Armee Oberkommando), in the Pas de Calais region, with the weaker 7th Army stationed at Normandy—a maneuver that would prove costly when D-day arrived.

Originally planned for May 1, 1944, the invasion date was set for dawn on one of three days—June 4, 5, or 6. Imperative that a combination of moonlight and high tide coincide in order to aid, respectively, the airborne and beach landings, Allied commanders chose June 5. However, unfavorable weather conditions caused Eisenhower to delay for 24 hours. The next optimal window of opportunity not until late July, Eisenhower made the decision to proceed with the invasion.

Just after midnight on June 6, the American 82nd and 101st and British 6th Airborne Divisions landed by parachute and glider on the Cotentin Peninsula behind German lines in support of the amphibious landings only a few hours away. Throughout the previous month the Allies had conducted a bombing campaign against key areas of northern France to destroy German communications.

In addition, French resistance, having received word of the impending invasion, sabotaged communication lines and railroads to delay German mobilization even more. The three airborne units, tasked with the further disruption of German capabilities, were to secure the flanks of the beaches, capture strategic bridges and causeways for Allied use, and destroy other key bridges that the German counterattack could utilize.

D-day casualties
D-day casualties

For the British 6th, assigned to capture the bridges spanning the Orne River and Caen Canal and protect the left flank of Sword Beach, mission execution was near flawless. Commanded by General Richard Gale, the division quickly completed its objectives within hours of landing in France and with very little mishap.

They had only to hold their position to await relief from the main attack force and keep German reinforcements—specifically the armored tank units— from advancing on the beaches. Unfortunately, the same could not be said for American paratroopers.

Due to poor visibility, German antiaircraft fire, and inexperienced pilots who had not flown in such conditions, both the 82nd and the 101st found themselves scattered across the peninsula. Nevertheless, per their training, units that failed to reach their designated zone were to carry out the missions assigned to the area in which they found themselves.

As a result, mixed units were able to assemble, organize, and achieve objectives on a limited scale. Ironically, German commanders became confused as to the Allies' intended target due to this situation, thus failing further to mobilize against the impending invasion.

As the airborne units carried out their missions, an Allied armada—the largest ever in history, which included close to 1,000 warships and 4,000 transport ships—made its way from assembly areas in southern England toward the Normandy coast. Having cancelled coastal patrols, the Germans were unaware of the Allied advance across the English Channel.

Nurses arrives in normandy

Around 5:00 a.m. a sustained Allied naval bombardment and assaults by bomber aircraft commenced against the German defenses on Normandy. The seaborne troops then began their approach to the five beaches by transport ships. The first ashore were elements of the U.S. 4th Division, landing at approximately 6:30 a.m. on Utah under intense German fire.

South of their target zone they faced lighter resistance than anticipated, thus minimizing expected casualties, and advanced rapidly up the beach to gain their objective. Only a few minutes later elements of the U.S. 1st and 29th Divisions landed at Omaha, where intact obstacles and fierce opposition bogged down subsequent waves of soldiers and equipment.

The congestion made the Americans easy targets for German gunners, resulting in the worst casualty rates of the entire invasion force—estimated at close to 95 percent for the first wave alone. Pinned by enemy positions atop the high bluffs that dominated the beach, many units suffered losses close to 60 percent and higher, which threatened the assault's success.

On the three other beaches the results were just as mixed. Landing around 7:30 a.m. on, respectively, Sword and Juno, the British 3rd Division, which also included French commandos, and the Canadian 3rd Division met typical conditions—obstacles that hindered their progress and strong opposition as well as the capacity to advance rapidly onward.

Thanks to continued naval bombardments that suppressed German defensive fire, both divisions were able to move inland by early afternoon. However, the British 50th Division, landing on Gold only a few minutes before, encountered an almost identical situation to what the Americans found on Omaha. Despite continual deployment of troops, the division could not secure the beach until after nightfall.

By the end of the day, close to 150,000 Allied troops had landed in France. In spite of heavy losses, although lower than expected, and the day's slow advance, which did not push inland as far as planned, the invasion was a dramatic success for the Allies.