Before Allied troops launched one of history's most important battles at the beaches of Normandy, France on June 6, 1944, they did nearly everything in their power to convince Nazi forces they weren't coming.

Across the water on the shores of England, Allied soldiers were building a fake army just about as big as the real thing. Operation Fortitude, as it was called, was comprised of inflatable tanks and trucks, fake track marks that feigned troop movements and even a constant stream of fake radio communication about a massive military strike to the North of Normandy that would never come. One expert Mashable spoke with called it the "poster child" of decoy operations, and while the world has seen nothing like it since, this kind of military deception is very much still a part of war.

"Essentially you have to do a lot of things differently, but the basic use of deception in warfare really hasn’t changed [since World War II]," Anthony Cordesman, former director of intelligence assessment in the Office of the Secretary of Defense, told Mashable. “You can always create different levels of deception.”

Two soldiers walk away after rolling an inflatable tank on its side. Image: Screenshot/History Channel

The inflatable distraction

The Allied D-Day deception began when military leaders from the United States, the United Kingdom and other nations realized that they would have to invade Nazi-occupied France if they were going to win the European war. But such an invasion could have been disastrous. If German forces were prepared for it, the whole of that nation's western army could have butchered the attack before it had a chance to start.

The Allies knew that, so they built a fake army of inflatable tanks, trucks and more that was comparable in size to their actual military might. The only difference is that this military was lined up across the sea from a point to the north of Normandy, near another potential landing site called Pas de Calais. The Allies also ran a stream of communications about the fake battle plans, hoping German intelligence officials would be listening, and they even mentioned that their most aggressive general, U.S. General Patton, was in command of the decoy force.

Below, you can watch a short History Channel video on the deception before D-Day.

Trickery worked then, and it still works now

Though multiple experts said Operation Fortitude remains the most elaborate fake army ruse in modern history, militaries still use hollowed-out decoy tanks and other constructions to fake out their battlefield opponents—though not without making a few modern tweaks.

"It’s still practiced today and it can still be effective today, especially against a country that’s relying on overhead imagery," Kevin Ryan, director of the Intelligence Project at the Belfer Center for Science and International Affairs, told Mashable.

In the late 1990s, when Serbia was trying to quell an armed resistance movement out of Kosovo—formerly a part of Serbia but now a partially recognized independent state—NATO forces swung the war by leading nearly a year-long a bombing campaign against Serbian troops. The Serbian government knew its air force didn't stand a chance competing in the sky, so experts said that part of its strategy was to build cardboard and inflatable tanks.

A soldier in England tips an inflatable military truck ahead of D-Day. Image: Screenshot/History Channel

But Michael Noonan, director of the national security program at the Foreign Policy Research Institute, told Mashable that Serbian soldiers also used to fill those fake tanks with water so they would heat up during the day, giving off a heat signal that NATO radar would pick up, making the decoys look all the more real.

During the Gulf War of the early 1990s, U.S. forces complained that their pilots were taking unnecessary risks because they were tasked with bombing fake Iraqi missile launchers in an effort to find the real things.

High-resolution satellite imagery has made it easier for militaries to track each other's troop movements, but experts said that armed forces that rely too much on overhead photos of enemy combatants are still in danger of falling victim to decoys.

“We have made advances in surveillance and imagery, but we have also made advances in creating replicas and so on," Ryan told Mashable. “We’re not doing 3D-printing of tanks, but you can make some pretty darn good cutouts."

Deception leading up to D-Day wasn't just about physical elements. The Allied misinformation campaign also led the German military to believe that an operation would take place at Pas de Calais instead of Normandy. That form of deception is even more prevalent in war today.

“You’ve seen some of that with some of the Russian actions in the Ukraine," Cordesman said.

Multiple experts said a Russian government campaign to influence Ukrainians and Crimeans over the last several months has helped it curry favor in those regions and cause chaos there without using anything close to its full military might.

By cutting off Ukrainian media and planting news stories about Ukrainian fascists trying to take over governments in Ukraine and Crimea, the Russian government was able to annex Crimea without undergoing a conventional war and has been able to aid various separatist movements in eastern Ukraine.

In the photo below, taken ahead of the March referendum in Crimea in which citizens there voted—under the gun—to join Russia, a billboard implies that not joining the Russian government will mean putting themselves under the rule of fascists like those of Nazi Germany during World War II.

It reads: March 16, we choose.

In a sense, experts said, a modern military's deceptive tactics are much the same as a military's tricks were during World War II, just with a few significant updates. The technology for and concept of cyberwarfare has allowed governments to take down websites of enemy nations with plausible deniability, for example, and simple technology today would have ruined decoy operations 70 years ago.

“You can’t hide in the same way you could back then," said Peter Singer, who has served as an advisor to a History Channel documentary on World War II and is the director of the Center for 21st Century Security and Intelligence at the Brookings Institution.

“Google Earth would have revealed the massive build-up across England in a way that the Germans would have dreamed about,” Singer said.

Militaries have to be more careful about how they set up decoys now, so as to avoid the plethora of satellite images. But trickery in warfare is at least as old as when Greek soldiers attacked the city of Troy by entering it inside a wooden horse that was presented as a gift to the Trojans, and technology has only changed the methodology.

“When you really look at it," Cordesman said, "warfare constantly changes, but it’s always the same.”

Bonus: Emotional veterans look back on D-Day