D-Day succeeded thanks to an ingenious design called the Mulberry Harbours

When Allied troops stormed the beaches at Normandy, France on June 6, 1944 – a bold invasion of Nazi-held territory that helped tip the balance of World War II – they were using a remarkable and entirely untested technology: artificial ports.

To stage what was then the largest seaborne assault in history, the American, British and Canadian armies needed to get at least 150,000 soldiers, military personnel and all their equipment ashore on day one of the invasion.

Reclaiming France’s coastline was just the first challenge. After that, Allied troops planned to fight their way across the fields of France to liberate Paris and, finally, onto Berlin, where they would converge with the Soviet army to defeat Hitler.

When Gen. Dwight Eisenhower and his advisers pressed for this ambitious invasion of Nazi-occupied France, British Prime Minister Winston Churchill was dubious.

Could it be done?

Such an operation would require more than a million soldiers – all equipped with weapons, ammunition, food and clothing – plus hundreds of thousands of vehicles, tents and medical personnel.

Getting so many people and materials from ship to shore while battling waves, tides and currents presented an enormous logistical challenge.

Churchill, recalling the failed marine campaign to capture Gallipoli during World War I, feared that Allied troops would get trapped on the beaches and be sitting ducks for the German soldiers awaiting atop Normandy’s cliffs.

So Churchill demanded that a team of engineers, scientists and military officers design a marine staging area that could actually support a successful operation.

The team’s solution was ingenious: two easy-to-assemble artificial ports where Allied ships could safely anchor to stage the massive operation.

As I write in my 2016 book on what became known as the “Mulberry Harbours,” each of these artificial ports consisted of artificial breakwaters – barriers against waves made up of sunken ships and huge concrete chambers.

Behind the circular breakwaters was a sophisticated system of floating piers anchored to the seabed.

All of these parts were towed 30 miles across the English Channel on D-Day from southern England, then sunk into place, about a mile off France’s northwest shore, the same day.

German planes doing air reconnaissance did spot the concrete chambers, which had been filled with air to make them float before they were sunk. But, according to my archival research, they had no idea what they were seeing or how these giant containers would be used.