The skies over Sannerville, France, were filled with paratroopers on Wednesday, in commemoration of the 75th anniversary of the Normandy landings on June 6, 1944. (Christopher Furlong/Getty Images)

Seventy-five years ago, Allied forces stormed the beaches of Normandy, marking the opening salvo of Operation Overlord. Secrecy was critical to the success of D-Day and, ultimately, the Allied victory in World War II.

Here’s how the Allies were able to keep the D-Day invasion secret from the Germans — and two big reasons maintaining this secrecy would be more difficult to achieve today.

A fake army

The Allies needed the Germans to shift their attention — and military forces — away from Normandy to have a better chance of success when landing on the beaches. The plan? Use a fake army to lure the Germans into focusing on another possible landing site.

The 1,100 men of the 23rd Headquarters Special Troops fabricated the Ghost Army, complete with inflatable tanks, rubber airplanes and sound recordings to round out the illusion. To lend credibility to the deception, Gen. George S. Patton, a top field commander, was put in charge of the unit.

During Operation Fortitude, the Ghost Army was tasked with convincing the Germans that the invasion would come at Pas de Calais, 150 miles northeast of Normandy and directly across the Strait of Dover — the most logical choice for an Allied invasion.

In the buildup to D-Day, Ghost Army operations successfully fooled Adolf Hitler into believing Normandy was not the primary landing site. As a result, German force levels in Normandy were lower than they would have been without the successful deception.

This wasn’t the only time the Allies tricked the Germans into thinking an invasion would occur elsewhere. The Allies deployed a similar tactic during Operation Mincemeat before the 1943 invasion of Sicily — albeit on a much smaller scale. The ploy also was used with success in North Africa earlier in the war.

Fake intelligence worked — thanks to strong personal relationships

Gathering accurate intelligence and disseminating false intelligence were both crucial to the overall war effort. But to be successful, those activities depended on relationships between individuals.

Fake intelligence even transpired between friends. In our recently published article in the Journal of Global Security Studies, Jonathan Brown, Alex Farrington and I uncovered new archival evidence showing that British officials cooked up and purposely faked intelligence about German intentions in Latin America.

British officials then shared this bogus information with American contacts with the explicit purpose of trying to bring the United States into the war before the Japanese attack on Pearl Harbor. The Americans readily accepted the information as legitimate, precisely because it came from trusted British sources with whom they had established positive relationships.

Allied spies also shared fabricated intelligence with the Germans, misdirecting them to maintain the secrecy of the Normandy invasion. The success of this deception once again hinged on the personal relationships between operatives and their targets.

Most famously, Spanish businessman Juan Pujol Garcia was a highly valued British double agent. Pujol Garcia — code name Agent Garbo — had spent years building up trust with his contacts in the German High Command.

After the June 6 invasion, Pujol Garcia told the Germans that Normandy was merely a “red herring” and that the real invasion was still to come at Pas de Calais. He even referenced Patton’s Ghost Army as evidence of the veracity of his information.

Hitler fell for the ruse and waited weeks to send reinforcements to Normandy, giving the Allies precious time to consolidate their foothold in France.

Technology vs. surprise

Deception certainly remains an integral part of warfare. In many ways, advances in technology create new forms of deception, such as cyberthreats and information warfare.

Yet surveillance capabilities have changed dramatically over the past 75 years, making it more difficult to maintain operational and tactical secrecy.

There are now thousands of reconnaissance and remote sensing satellites in space. These satellites not only provide governments with detailed images of on-the-ground operations, but also collect data on other critical signals of military action, including communications.

As Stephen Walt explains, these satellites “can monitor global hot spots on a more-or-less real-time basis and let leaders know if forces are being moved and prepared for combat.”

In 2017, imagery from microsatellites detected military exercises in Venezuela. Satellite imagery also revealed violations of the Safe Demilitarized Border Zone between Sudan and South Sudan in 2012.

Advances in drone surveillance technology further provide fine-grained detail about opposing military forces, including troop movements. Surveillance drones, for example, can track al-Qaeda operatives and support counternarcotics operations.

'Loose lips sink ships’ — but so can social media

Social media and other digital technologies further threaten the secrecy of large-scale military operations. Social media offers new ways to disseminate false information but also opens up operational and tactical vulnerabilities.

Keeping the D-Day plans under wraps was a difficult task and the secret was almost blown on several occasions, according to some sources.

But today’s soldiers can be careless about how they use social media, revealing crucial information about troop movements. In 2014, a Russian soldier posted his status on social media as “To Ukraine!” Another Russian soldier uploaded photos in 2015 that revealed he was in Syria.

These instances, and others, directly undermined Russia’s claims it had no military involvement in Ukraine and Syria at the time. Russian lawmakers recently voted to restrict military personnel from using smartphones or posting information about their service activities online.

Personal fitness trackers can also reveal the location of military personnel. In 2017, the GPS tracking company Strava released its Global Heat Map compiled from location and movement data from personal fitness trackers, including the widely popular Fitbit.

Strava’s map was linked to “U.S. military forward operation bases in Afghanistan, Turkish military patrols in Syria, and a possible guard patrol in the Russian operating area of Syria.” The U.S. Department of Defense considered banning cellphones and personal fitness trackers from military facilities in response.

Between satellites and smartphone apps, it’s almost impossible to imagine pulling off an operation like D-Day in 2019. It was hard enough to keep secret in 1944.

Danielle Lupton (@ProfLupton) is an assistant professor of political science at Colgate University. She is the author of “Reputation for Resolve: How Leaders Signal Determination in International Politics (Cornell University Press, forthcoming).