Though flamethrowers aren’t entirely banned, you can’t use them to fry your enemies, according to Protocol III of the Convention on Certain Conventional Weapons. This clause prohibits the use of incendiary weapons on people. You can, however, use them to clear foliage.

Surprisingly thermobaric bombs are legal. They create a plasma cloud that has a long dwell time and the resultant overpressure wave creates fearsome tissue damage.