Typically used in naval battles, Greek Fire was effectively a rudimentary flame-thrower that could supposedly continue burning while on water.  

Some historians believe that the weapon could be ignited on contact with water, so likely had a compound of naphtha and quicklime. Whatever the formula, the advantage it gave the Byzantine Empire is unquestionable. Thanks to the successful salvation of Constantinople from the first and second Arab sieges, they secured the Empire’s survival. It may not be the first incendiary weapon in history, but Greek Fire is definitely one of the most historically significant.

'Byzantine - Greek Fire' by Patrick Gray (Source: Flickr) is licensed under CC BY 2.0 (original image resized, cropped and altered)