The compound known as “Greek fire” was the Byzantine navy’s secret weapon. It was a flammable liquid hydrocarbon that could be blown through tubes, thrown in sealed pots, or poured on the heads of soldiers attacking fortifications. Its precise composition was a state secret; and this was kept so well that even now we are not certain of its ingredients.
I was reminded of it recently from this passage in Pliny’s Historia Naturalis, which I accidentally came across while writing a recent article on the subject of St. Elmo’s Fire. Pliny’s description of it (II.108-109) makes it sound very much like modern napalm:
In the Commagene city of Samosata [in what is now Iraq] there is a marsh which oozes a flaming mud called “pitch.” It adheres to solid surfaces with which it comes into contact; and having been touched, it follows the person who runs away. They used it to defend the city walls when Lucullus laid siege to it; and the soldiers were thus burned by their own weapons. It even grows more intense when water is thrown on it; trial and error have demonstrated that it can only be extinguished by soil.
Naphtha is of a similar nature. It is called by this name around Babylon and in the area of Astacus in Parthia; it is flowing bitumen in liquid form. It has a strong connection with fire; it immediately moves in the direction of fire wherever it shows itself…
The medieval sources state that the secret of Greek fire was brought to Byzantium by a chemist named Callinicus, a native of Heliopolis in Syria, who defected from the service of the Islamic caliphs to work for the Romans. According to the historian Edward Gibbon, the “principal ingredient” of the mixture was naphtha; to this was added pitch from evergreen fir trees and sulphur. It is even possible that cedar oil taken from Lebanon was used also.
The liquid, when it came into contact with air, would produce a loud explosion and set on fire anything that it touched. Nothing causes men to panic quite like and inextinguishable fire. It was also flexible in deployment: it could be dumped on invaders during a siege, or used in naval combat by being flung in earthen pots or blown through tubes.
Gibbon says, “[it] was most commonly blown through tubes of copper, which were planted in the prow of a galley, and fancifully shaped into the mouths of savage monsters, that seemed to vomit a stream of liquid and consuming fire.”
We do not know much about how it was used tactically during combat. As far as I have been able to determine, no examples survive of the actual “flame-throwing” tube mechanism used in battles. So we are forced to rely on contemporary accounts and reasonable inferences.
In the age of galley warfare, ships manned by rowers would maneuver to try to position themselves so as to throw out grappling irons or hooks against opposing vessels. They were then boarded by armed men. Greek fire was a defense against this; any ship caught within the range of the dispensing tube would be hosed down with the deadly concoction. The effect must have been terrifying for enemy sailors and naval infantry; just a handful of “fire ships” would have been able to deter whole squadrons of enemy vessels.
The secret of Greek fire was well-kept. The rigors of religious and penal sanction functioned as effective deterrents to anyone who dared reveal anything about it. But as often happens with such things over time, the secret gradually leaked out. It was put to use by both Arabs and Greeks in the Near East, until projectile weapons using gunpowder replaced it in the fourteenth and fifteenth centuries.
 In urbe Commagenes Samosata stagnum est emittens limum (maltham vocant) flagrantem cum quid attigit solidi, adhaeret; praeterea tactus et sequitur fugientes. Sic defendere muros oppugnante Lucullo, flagrabatque miles armis suis, aquis etiam accenditur; terra tantum restingui docuere experimenta. Similis est natura naphthae: ita appellatur circa Babylonem et in Astacenis Parthiae profluens bituminis liquidi modo…etc., etc.