One of the most intimidating standard-issue weapon fielded to troops is, without a doubt, the flamethrower. Yes, bullets are intimidating, but nothing shocks and terrifies the primitive side of our human brains like a wave of fire surging toward you.
In contemporary warfare, the use of flamethrowers has tapered off in favor of more accurate weapons. Contrary to popular belief, they are not outlawed by the Geneva Convention — they just can't be used anywhere near civilians. The most notable modern example of a flamethrower being used against another person was in 2014, when it was used as an execution tool by North Korea against its Deputy Minister of Public Safety.
The flamethrower, as we know it, was first created by Germany in 1901 and was known as the flammenwerfer. The flamethrower would find immense popularity among troops in the trenches of WWI, the all-out war of WWII, and the forests and jungles of the Korean and Vietnam Wars.
However, the ancestor of what we call the "flamethrower" today got its start early in history with the Byzantine Empire. In 672, Crusader navies would spew a napalm-like substance, called "Greek Fire," on their enemies. The actual composition of Greek Fire was a closely guarded secret that is now lost to time, but scholars generally agree that pine resin was used to make it sticky.
As technology evolved, Greek Fire was then launched out through a hand siphon that a troop could carry into battle. This was called a "cheirosiphon." Crusaders would station a hand siphon atop a ladder or wall and spray the Greek Fire down, raining chaos onto their enemy.
In the East, China invented their own version in 919, during a time known as the Five Dynasties and Ten Kingdoms period. The Pen Huo Qi worked nearly identically to the Crusaders' flamethower, but it was more elaborate and was made to resemble metal dragons breathing fire.
Outside of the Crusaders, Vikings may have also created their own version of Greek Fire in 1041 (albeit with a different name) after they laid siege on Constantinople. The Saga of Yngvar the Traveler tells the story of a man (Ingvar) as he learns the art of flame-throwing — because apparently regular vikings weren't terrifying enough.
When guns and gunpowder became the dominant weapon on the battlefield, the comparatively short range of flamethrowers made it less appealing — but it wasn't ever forgotten. Threats of using Greek Fire even persisted through the American Civil War.