Flamethrowers are subject to the same restrictions as any other incendiary weapon. These restrictions are that they cannot be used against civilians, or against military forces in close proximity to civilians. Any other use is perfectly legal under the laws of war. Flamethrowers have mostly fallen out of favor (replaced by napalm-filled rocket launchers such as the M202 FLASH) because they are very short ranged and the large fuel tank has a habit of igniting when struck by a bullet.


The "undue suffering" clauses of the conventions are directed primarily at things like blinding lasers, triangular knives, and similar weapons that are explicitly intended to not just take somebody out of the fight but mess them up so bad that the enemy has to direct massive resources to treat them.