Greek fire, not fully understood even to this day, is the defining icon of the Byzantine military. Primarily used in naval warfare, it was a combustible liquid fired through a siphon onto the decks of enemy ships. A terrifying spectacle, the liquid ignited with a sound like thunder, and burned even on water. The only sure way to extinguish the flame was to throw sand over it.
In addition to the shipboard “flame cannon,” Greek fire was also employed on land; soldiers were (rarely) equipped with bulky hand siphons which served as medieval flamethrowers. Accounts also exist of hand grenades being used by the Byzantine army.
The formula for Greek fire was a closely-guarded state secret. Indeed, by the time of the Fourth Crusade, even the Byzantines had all but forgotten the method of its creation, and some historians theorize that recent losses along the coast denied them access to crude oil, believed to be a primary ingredient. However, accounts exist of hastily-thrown-together fireships sailing to defend Constantinople’s harbor from the Venetian fleet.