The following are three famous forms of liquid fire throughout military history in chronological order: Greek Fire (used at sea), Flame Throwers (used on land), Napalm (used from the air).
- Greek Fire: Invented by Callinicus in 673AD, it was one of the reasons why the Byzantine Empire survived. Impossible to extinguish, incredibly adhesive, and ignited by water, it was extremely useful in naval battles and in defending the city of Constantinople. For these reasons, the Byzantine Empire always tried to keep Greek Fire their own secret. In 1453, the secret was lost forever and we still don't know how to replicate it.
- Flame Throwers: Invented by Richard Fiedler in 1901, the most common form is the backpack flamethrower. The backpack flamethrower has two canisters: one for compressed flammable gas, and the other for flammable liquid. They were used especially in WWII to eradicate Japanese from tunnel systems, but they are effective against all kinds of fortification. However, they are extremely dangerous to the user, and only useful in certain circumstances.
- Napalm: First used in WWII, it was a key part of air power during the Korean War and the Vietnam War. In the latter, it was especially useful against entrenched Vietcong; in the former, it was especially useful against tanks. However, it is considered an inhumane weapon and in 1980, the UN decreed that it shouldn't be used against civilians.
For more information, read:
Military History Magazine (April 2006): The Lost Secret of Greek Fire
ABC-Clio: US at War: Napalm
http://science.howstuffworks.com/flamethrower.htm Military History Podcast is sponsored by Armchair General Magazine