Flamethrowers are popular today in many video games, but when they were first invented, they were formidable weapons on the battlefield.  Obviously, being flamed by a flamethrower hurts a lot, especially for people wearing armor made of either fabric (it catches on fire) or metal (it melts).  Given that the invaders China was facing at the time of this invention were probably not enlightened on the theories of insulation, it is unlikely that any of them could defend against this weapon. As a result, flamethrowers could keep invaders from even having a chance to enter China.

Date of Invention:
Around 900 AD

Inventor: 
Unknown

Importance to Chinese civilization:
Was a powerful weapon in the Chinese arsenal

Importance to other civilizations:
Became a popular method of melting people in many popular combat video games