The legendary amphibious landing of World War II was the result of multiple years of intense planning and argument by the American and British militaries. It changed the course of history forever. 

Now, almost eighty years ago, over 23,000 airborne troops landed in Nazi-occupied France with 132,450 Allied forces crossing the channel via sea; 6,833 vessels—including 1,213 warships—took part with over 14,000 sorties flown during the previous evening and D-Day itself; five beach areas were devised, with the First U.S. Army landing on two and the Second British Army landing on the other three. By the end of August, over 2 million men, 3 million tons of supplies and stores, and almost half a million vehicles had landed in Normandy.

But what led up to this moment was multiple years in the making. After the Dunkirk evacuation and the subsequent fall of France, Great Britain and its empire gallantly continued a lonely, year-long struggle as the only major power fighting German Nazism and Italian Fascism—until the Nazi invasion of the Soviet Union. Although no clear victory was in sight and with the Britannic world overstretched—battling over the air of Western Europe, on land in North Africa, and at sea in the Atlantic and Mediterranean—Churchill had always planned for a later liberation of Western Europe.

In fact, after the United States’ direct entry into the war, following Japan’s attack on Pearl Harbor, Churchill crossed the Atlantic Ocean to meet with President Franklin Roosevelt for the Arcadia Conference. Arcadia set in motion the strategic and operational process that would eventually culminate in D-Day. It reaffirmed the position taken earlier in 1941 by British and American planners: should the United States enter the war, the first primary objective would be to defeat Germany.

That December, during the Arcadia Conference, Churchill presented the American Joint Chiefs of Staff with his general strategic concept to defeat Germany—agreed upon by the British Chiefs. It simplified as five key points: enact naval blockades against the Axis; heavy bombing campaigns against Germany; propaganda designed to incite rebellion in occupied countries and break German morale; a series of peripheral landings across the European coastline by small armored units; and then the “final assault on the German citadel.”

Such a plan clearly drew upon British history. In the words of American rear-admiral Samuel Eliot Morison, “They remembered they had got at Napoleon by the back door of the Iberian Peninsula, and that direct-attack strategy in World War I had cost them a million men killed and two million wounded.”

The Americans didn’t view such a plan favorably. While they agreed that such actions would weaken Germany, what was really needed was one powerful and major operation, meeting the Germans head-on. Many within the U.S. government were eager to open a second front as soon as possible.

However, psychologically, the British were met with two problems: first, the memory of the static warfare from the Great War. The fear of such a repeat was common among many in the upper echelons of British war planning. Secondly, Britain had already been kicked off the European continent three times by this point: in Norway, during the Fall of France—notably at Dunkirk—and in Greece.

Such psychology remained an essential part of the developments leading up to D-Day.

1942 saw grand movements in military decision-making. On March 27, 1942, Roosevelt was presented with a plan drafted as the Marshall Memorandum after Chief of Staff General George Marshall. Subsequently approved by the president on April 1, it was immediately sent across the Atlantic with Marshall and Roosevelt’s adviser, Harry Hopkins.

The plan outlined three operational phases: First, prepare for the invasion. Logistically, this was already in motion with Operation Bolero—the Combined Allied Chiefs of Staff planning for the transportation and supply of American forces and materiel into the UK. Part of the document’s preparatory measures included Operation Sledgehammer, an emergency cross-channel invasion of the Cotentin Peninsula to take pressure off the Soviet Union if it faced imminent collapse.

The second phase was a cross-channel invasion codenamed “Roundup.” The final stage was the advance into Germany after consolidating the beachhead in France.

The British wholly rejected Sledgehammer as the Americans could have only provided “token forces” for such an operation at the time. Nonetheless, the Western Allies kept it “on the books” to satisfy Soviet demands for a second front.

Churchill instead argued for an invasion of North Africa in 1942, an idea with which Roosevelt agreed. This provided the U.S. forces with a direct opportunity to fight the Germans and Italians while securing Britain’s need for victory in Africa.

General Marshall, having failed to convince the British on Sledgehammer, agreed to the North African alternative—though he would later admit that his thoughts on the success of Sledgehammer were premature, especially given that D-Day required Allied mastery of the air, something that was lacking in 1942.

In June 1942, General Marshal was eager and fixated on Bolero. Logistics for Bolero remained an ever-present concern—and although America was eager to press onto the continent, it had caused some disagreements within Washington. This is perhaps best explained by Chief of the Imperial General Staff Alan Brooke, who relayed the dissensions he witnessed with the Combined Chiefs of Staff back to Westminster. Written in the confidential annex of a War Cabinet meeting:

Other very high quarters in Washington were, however, apprehensive that if we concentrated on BOLERO to the exclusion of any other projects, either in 1942 or 1943, there was a danger that, if BOLERO was not practicable, large bodies of American troops would be locked up in Great Britain and remain indefinitely inactive.

Logistics would continue to compound problems. By November 1942, Bolero’s slowdown sent a somber message. As General Eisenhower told Churchill, they could not attempt any feasible and meaningful cross-channel invasion until 1944.

At the Casablanca Conference in January 1943, the Allies reaffirmed their commitment to a cross-channel invasion and, as such, to establish a joint Anglo-American planning staff in London.

Subsequently, the Washington Conference in May 1943, codenamed Trident, enacted the necessary compromise for D-Day. The British agreed to commit to a target date of May 1, 1944, for a cross-channel invasion. The Americans, in turn, agreed that such operations were best left until after the conquest of Sicily, continuing the commitments in the Mediterranean.

The Quebec Conference in August 1943 provided the next required stepping stone for D-Day: an American Allied Supreme Commander. Churchill had initiallypromised Brooke the position of supreme commander. Still, given the increasing number of American units likely involved in a cross-channel landing, it became imperative that a U.S. officer held the position. Thus, on December 7, 1943, Roosevelt met with Eisenhower and told him, “Well, Ike, you are going to command OVERLORD”—the codename for the future battle to come.

The lead-up to D-Day was complex and changing. The Germans were aware of an impending invasion based on their many intelligence reports and successfully tapping a telephone conversation between Roosevelt and Churchill in February 1944. Though unaware of this tap, the Allies nonetheless had a plan to deceive the Germans over the location of the invasion: Operation Fortitude South.

It ultimately fed and nurtured the Nazi preconception that the main Allied invasion was to be at the Pas-de-Calais, partly as it seemed the most logical military. Hence, General Bradley referred to it as “the biggest hoax of the war.” The perceived threat of an invasion of the Pas-de-Calais proved invaluable to the Allied cause. Best described by Eisenhower in his later report sent to the Combined Chiefs of Staff on the operations in Europe of the Allied Expeditionary Force:

The German Fifteenth Army, which, if committed to battle in June or July, might possibly have defeated us by sheer weight of numbers, remained inoperative during the critical period of the campaign, and only when the breakthrough had been achieved were its infantry divisions brought west across the Seine-too late to have any effect upon the course of victory.

From February to April 1944, the Fifteenth Army, north of the Seine, increasedfrom ten to fifteen divisions. However, Nazi “intuition” on the dangers of an undefended Normandy led them to enact multiple division changes. The result was that in addition to their twelve SS Panzer Divisions stationed at Lisieux, three Panzer Divisions (between the Seine and Loire rivers) that could quickly respond to defend Normandy were brought.

Originally scheduled for May, D-Day was delayed a month to procure more LSTs (Landing Ships for Tanks). This was imperative as, in the early hours of April 28, 1944, eight American LSTs were conducting invasion drills off the English coast by Lyme Bay. Intercepted by German torpedoes, two were destroyed, and two were damaged—one beyond repair. The margins for available LSTs were so thin that the loss of three of them threatened the entire viability of D-Day.

The weather affected two principal parts of the operation: D-Day itself, the day the invasion was to take place, and the H-Hour—the hour the Allied assault craft needed to ‘touch down’ onto the beaches. Intended for June 5, Eisenhower again delayed the invasion by a day, but this time due to poor weather.