Greek fire is an incendiary weapon which appears to have been developed around the seventh century. It is named after the Byzantine Greeks, who were especially fond of using it in battle, although it was also employed by the Arabs, Chinese, and Mongols, among others. This weapon was extremely devastating, striking fear into the hearts of the enemy and effectively mowing down troops, ships, and other weapons of war. Numerous testimonials from the period speak to the power of this weapon.
Intriguingly, the formula for Greek fire was kept so secret that it quickly became lost, and today no one is exactly sure what it was. It appears to behave somewhat like napalm and other modern incendiaries, in the sense that it was extremely difficult to put out. It appeared to ignite in water, and pouring water on it caused the fire to grow even larger, leading some people to believe that it may be related to thermite.
Some theories for the composition of Greek fire include ingredients like petroleum, which was known to people during this period, along with naptha, quicklime, sulfur, niter, and saltpeter. Many of these ingredients are used in contemporary explosives, testifying to their power, and they would have been available and known to at least a limited section of humanity during this period in history. The development of Greek fire is probably closely related to alchemy, the ancient precursor to chemistry.
As you might imagine, Greek fire was an extremely effective and scary weapon. The Greeks used it to create fire ships, setting empty ships on fire and putting them on course for the enemy, and it was also used to make incendiary bombs which could be hurled with catapults onto other ships. The fire was also apparently held in large cauldrons and directed with a hose, preventing unwanted boarding of ships and scaling of walls.
Many people wrote about Greek fire with large amounts of fear and respect. This incendiary weapon certainly greatly contributed to a number of Byzantine military victories, and some people have likened it to the atom bomb, suggesting that just as the atom bomb was the most devastating weapon of the second millennium, Greek fire was the most effective and terrifying of the first. Several attempts have been made to replicate this weapon, using ingredients which would have been accessible to its inventors, but no satisfactory formula has been created.