Greek fire
Originally an incendiary weapon adopted from the Greeks in 7th century AD, the Greek fire "is one of the very few contrivances whose gruesome effectiveness was noted by" many sources. Roman innovators made this already lethal weapon even more deadly. Its nature is often described as a "precursor to napalm". Military strategists often put the weapon to good use during naval battles, and the ingredients to its construction "remained a closely guarded military secret". Despite this, the devastation caused by Greek fire in combat is indisputable.