5 Things You Might Not Know About D-Day > U.S. Department of Defense > History

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D-Day. The invasion of Normandy. Operation Overlord. It goes by different names, but we’ve all heard of it through history class, grandparents, the news, or shows like “Band of Brothers.”

June 6, 1944 was the day that more than 160,000 Allied forces landed in Nazi-occupied France in the largest air, land and sea invasion ever executed. It ended in heavy casualties – over 9,000 Allied troops were killed or wounded in those first 24 hours – but D-Day is widely seen as the successful beginning of the end of Hitler’s tyrannical rule.

The bravery of the paratroopers and soldiers who stormed Normandy that day is well known, but there’s a lot you might not know about D-Day. Here are a few of those nuggets.

Why is it called D-Day

Do you really know what D-Day means? Apparently, this is the most frequently asked question at the National WWII Museum, but the answer is not too simple. Many experts have differing opinions, including that the D simply stood for “day”, a code used for any major military operation. Others said it was just an alliteration, like “H-Hour”, when a military assault begins.

While the true meaning is up for debate, we’ll follow what US General Dwight D. Eisenhower said about it through his executive aide, Brig. General Robert Schultz: “Know that every amphibious operation has a ‘start date’; this is why the abbreviated term “D-Day” is used. He said there were actually several other D-Days during the war – Normandy was just the biggest and best known.

D-Day was originally set for 1 day earlier

Many weather-related requirements were needed to make D-Day a success. The days had to be long for maximum use of air power; a nearly full moon was needed to help guide ships and airborne troops; and the tides had to be strong enough to expose beach obstacles at low tide and to float landing craft filled with supplies far down the beach at high tide. H-Hour was also crucial in that it relied on the rise of those tides at that time. There also had to be an hour of daylight just before for bombing accuracy.

Only nine days in May and June seemed to meet these requirements, so the commanders finally settled in on June 5; however, with forecasts showing a short window of good weather that day, General Eisenhower decided at the last minute to shift D-Day to the early hours of June 6.

We only stormed 2 of the 5 beaches

Stories of how American troops stormed the beaches of Normandy have been legendary for years, with the names of Omaha Beach and Utah Beach standing out in people’s minds. But the invasion stretched over 50 miles of land, so we couldn’t do it alone. Three other beach invasions by Allied troops occurred simultaneously: Britain and a few smaller forces stormed the beaches at Gold and Sword, while the Canadians took Juno Beach.

It was almost a failure

While the ultimate goal of liberating France and ousting the Germans came true, a lot went wrong on D-Day, especially for the Americans, who were the first to launch the invasion.

Thousands of American paratroopers died when dropped behind enemy lines at Utah Beach, having been knocked from the sky by enemy fire or weighed down and drowned in flooded swamps. Many also missed their landing spots, as did the sea forces, which landed more than a mile from their intended destination, thanks to strong currents.

The Omaha offensive turned out to be the bloodiest of the day, largely because Army intelligence underestimated the German stronghold there. Rough waves caused huge problems for amphibious tanks launched at sea; only two of the 29 reached the shore, while many of the infantry who stormed the boats were shot down by the Germans. General Omar Bradley, who led the Omaha forces, almost considered abandoning the operation.

Somehow, however, both sectors of American troops managed to advance their positions for overall success.

Decoding “Enigma” helped us win

Decode the great German code machine known as Enigma, then keeping this decoding device a secret, is one of the most brilliant strategies to come out of World War II.

Long story short, since radio was the standard communication of the time, both the Allies and the Axis powers needed machines to turn military blueprints into secret codes. The Germans had Enigma, which was thought to be unbreakable – until it wasn’t. At the start of the war, a team of Polish and British experts, led by Alan Turingwhose life and work are depicted in the Oscar-winning film “The Imitation Game” – cracked this code through what became the basis of the modern computer.

Instead of telling the world about it, however, executives believed the device would be more useful if kept secret. So for years German plans were hampered by the decrypted messages, including D-Day. Officials said German codes intercepted before D-Day accurately identified almost all German combat units in the Normandy area. On D-Day itself, it also helped Allied commanders be informed of their troops’ progress faster than through their own communication channels.

Breaking German and then Japanese codes proved to be a huge advantage for the Allies. Although controversial for its secrecy, the decoding process has been widely credited with saving hundreds of thousands of lives and shortening the war by nearly two years.

Oh, and by the way, the US military has developed its own superior code machine – SIGABA — before going to war. Nobody could break that one.

If you didn’t know any of these things before today, now you do! Either way, be sure to remember those who gave their lives that day to help secure a better future for all of us.

(Editor’s note: This article was originally published on June 3, 2016.)

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