#3. Stealth Bombers Were Invented by the Nazis
There's a reason allied countries were all fighting over Nazi scientists after World War II ended: They were really good at coming up with weapons systems that at the time seemed like goddamned science fiction. For instance, although the U.S. military didn't have stealth aircraft until the 1980s, the Nazis were actually developing the technology four decades earlier. This was the closing years of the war, when the Nazis came to realize that things weren't going their way and they started throwing money at any crazy-ass idea that might give them an advantage. The result was the Horten Ho 229 ...
...which looked remarkably similar to the state-of-the-art B2 stealth bomber:
U.S. Air Force
Half cutting-edge aeronautic technology, half Batarang.
Sadly for the Nazis (but happily for everyone else), the aircraft came too late into the action to make any difference at all. After a few test flights, it was captured by American troops and never used in combat ... which is probably a good thing, since tests done on it after the war showed that the Horten Ho 229 was in fact goddamned invisible to radar. It could have basically waltzed right up to London, kicked it in the face, and walked away before the RAF had even figured out what had happened.
#2. Kevlar Was Used by the Ancient Greeks
If you've never had to wear bullet-stopping body armor, you should probably consider yourself fortunate. But if you ever put some on, you'll be surprised -- where you'd expect a heavy metal plate or something else that seems appropriate for the name "body armor" you'll instead find just a thick core of light fabric that doesn't seem capable of stopping anything more than a slingshot. That's Kevlar, a material invented in the 1960s that involves bundling tons of layers of thin fabric that, thanks to the miracle of science, dissipates the energy of projectiles wonderfully.
But while many a police officer and soldier has probably taken time to thank the technological wizards at DuPont for inventing the stuff, in reality the invention goes back way, way further than that. A material remarkably similar to Kevlar is known to have been used by Alexander the Great. And even before that, historians have found references to linen body armor going back around 3,000 years, as far back as the time of Homer's The Iliad.
Although modern depictions like 300 show the ancient Greeks marching into battle with nothing but their naked, rippling abs to protect them against a storm of arrows, the Greek soldiers were wearing a lightweight armor they called linothorax.
Via University of Wisconsin Green Bay
Whereas their necks were protected by beards that look capable of stopping a bazooka.
Nobody really knows exactly what linothorax was made from, because unlike metal armor such as bronze breastplates, the lightweight stuff rotted away over the millennia. Luckily, historians and/or LARPers with Ph.D.s at the University of Wisconsin-Green Bay have found a way to recreate the ancient armor using methods and materials that would have been available at the time. Like Kevlar, it's a matter of taking a tough, thin material (linen) and fusing it together in layers. And it totally works (for the exact same reason as Kevlar) and was superior to bronze armor -- not only did it stop arrows, but it didn't need to be custom made for soldiers. How do we know that? Because they tested it by firing arrows at their graduate students.
University of Wisconsin Green Bay
Turnout spiked when the "puncture a lung: automatic A" policy was enacted.
By the time of Alexander the Great, linothorax was standard armor for Greek hoplites and worn all throughout the Mediterranean. The famous mosaic at Pompeii also shows Alexander wearing it as he suavely tramples the hopes of the Persian King Darius at the Battle of Issus.
Even his horse looks skeptical.
Linothorax gradually got phased out in the Roman Empire, but that didn't stop other fabric vests from coming into vogue. In the 19th century, a bunch of guys came up with the idea of layering silk to create body armor, which also totally worked as long as you were rich as hell and could afford it. If nothing else, when you died you'd look fancy as shit.
#1. Tanks Were Used in the 1400s
As soon as tanks began rolling onto the battlefield, warfare was changed forever. And we all know that was the First World War, right? You didn't see primitive tanks in, say, the Revolutionary War. What would that even look like? A cannon tied to a bear?
Actually, you can go back even further than that. A member of a Czech military faction named Jan Zizka beat everyone to the punch by 500 years. And yes, it looked ridiculous:
Ludek, via Wikipedia
But you can't argue with the unstoppable fury of horse-drawn plywood.
Sure, they were made out of wood instead of steel, but Zizka's war machines that rolled out in the 1400s were tanks in every other respect -- a box with wheels, an obnoxiously large cannon, and a bunch of guys in it. And they could do shit that modern tanks can't do today. For instance, they could link them all up to create an impenetrable wall of death and destruction.
Depicted here with complete historical accuracy.
With the ability to move up to 25 miles a day, Zizka's "battle wagons" were incredibly successful, leading to over 50 victories in a 14-year period. And during this whole time, no one even bothered to think to possibly build their own versions of these things. With enemies apparently content to throw up white flags, Zizka owned any battlefield he found himself fighting on.
Related Reading: Hey, and while we're nerding out over weapons why not check out these plasma technologies that put every video game Maguffin to shame. If you prefer your war machines with a side of "terrible idea", the Bob Semple tank may be right for you. Prefer to see the functioning real-world answers to sci-fi weapons? We can help with that, too.