71 years ago today, Allied forces stormed the beaches of Normandy. Operation Overlord was the largest amphibious invasion to date, and created a foothold in Europe. The key to winning? Careful planning and training. And while the invasion was a success, it was a risky gamble that could have gone disastrously wrong.

Following the start of the Second World War, the Allies were effectively shut out of Europe by the Germany military. A cross-channel assault was expected by both sides, and Germany embarked on a massive program to fortify the Atlantic coast with defenses to repel invaders. As the coalition of allies regrouped, they began fighting in Northern Africa, Eastern Europe and in the Mediterranean, in order to chip away at the German lines.

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In 1943, planning began in earnest for a cross channel invasion, when the heads of the major allied powers, Roosevelt, Churchill and Stalin agreed to open a second front against the German military, to relieve pressure on Soviet Forces fighting in the East. Planners began looking for likely invasion points: Brittany, the Cotentin Peninsula, Normandy, and Pas de Calais. Pas de Calais was rejected because of its location: its proximity to Great Britain meant that it was heavily fortified. Brittany and the Cotentin Peninsula were each rejected because of the ease to which each location could be cut off. Normandy remained, even as several challenges would have to be overcome.

The invasion would eventually include thirty-nine divisions from the United States, Great Britain, Canada, Poland, and France. The enormous scale of the operation required enormous resources massing in England: additional landing craft, airplanes and armored vehicles were required, pushing the date of the invasion back by a month.

With the location set, the Allies began planning for the assault. Army intelligence began to intensely study the area: the region’s tides, topography, weather and terrain were examined to determine the best places to land soldiers, but also the timing: too early, and soldiers would be crossing vast tital flats. Too late, and landing craft might run into defenses hidden under the water. Planners also took the lunar cycle into consideration: a full moon would provide troops landing overnight with some light to work with once they landed.

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June 4th was selected as a potential landing date, but rough weather forced planners to hold their plans. Meteorologists predicted that they would see a break in the weather on the 6th, the last possible day: anything longer would force them to wait for the next cycle of tides, at the end of June. On the other side, German meteorologists, without access to the Atlantic ocean, predicted storms throughout early June, prompting much of the German command in the region to participate in war games, thinking that an invasion of any sort would be impossible. On June 5th, Supreme Commander Eisenhower gave the order, and the invasion was underway, with Allied forces storming the beaches starting in the early morning hours of June 6th.

Once captured, Normandy would have to function as a foothold and port for a massive military force. Because the area wasn’t home to port facilities already, an artificial one had to be constructed. Prefabricated parts for the facilities were constructed in England, and brought over. The idea was proposed by Hugh Iorys Hughes, who was made Naval chief of staff for the invasion planners. In the days after the beaches were taken, construction commenced on the harbors, which were put into place shortly thereafter. One was destroyed in a storm on June 19th, but over the remainder of the war, thousands of vehicles, millions of soldiers and tons of materials were transported over the new facilities.

The key to the successes of Normandy wasn’t only due to the skills and perseverance of the Allied soldiers who landed on June 6th. They were aided by the preparation that went into designing the invasion, which took into consideration the weather, tides, terrain and supplies needed to keep a million-strong force moving forward. Without it, the invasion would have likely failed, prolonging the war for months or even years.

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Photo credit: National Archives and Records Administration