For centuries the Byzantine Empire used Greek Fire during naval battles. This gave them a great technological advantage, and was responsible for many key military victories such as the salvation of Constantinople (in 1203) from two Arab sieges.
Incendiary weapons were used in warfare for centuries before the invention of Greek Fire, but what set the Byzantine Greek Fire apart was its unique formula - which would continue burning even underwater - and the use of pressurized siphons to project the liquid onto the enemy ships. Some consider Greek Fire the predecessor of modern napalm.
The ingredients, the processes of manufacturing, and the precise deployment of Greek Fire were extremely well-guarded military secrets. Knowledge and fabrication of the substance were highly divided, with operators and technicians aware of the secrets of only one component, ensuring that no enemy could gain knowledge of it in its entirety. For instance, some Bulgarians captured 36 Greek Fire siphons in the early 8th century, but were unable to make any use of them.