Though flamethrowers aren’t entirely banned, you can’t use them to fry your enemies, according to Protocol III of the Geneva Convention on Certain Conventional Weapons. This clause prohibits the use of incendiary weapons on people. You can, however, use them to clear foliage.
You might not think of pepper spray as a weapon for warfare. But because of the Hague Convention’s ruling on aerosol chemical weapons that disrupt breathing, you can’t bring your pocket mace to the battlefield … not that you should. It’s not exactly an effective weapon, but more like an unpleasant deterrent.
Militaries are no longer allowed to set up land mines that can’t be detected by x-ray. Under Protocol I of the Geneva Convention on Certain Conventional Weapons, there is a requirement that all weapons must use metallic fragments that can be seen via x-ray. In addition, mines placed outside of fenced and cordoned areas are required to use self-destruct mechanisms set to go off after a certain period of time. There is also an ongoing campaign to ban the use of land mines internationally through the Ottawa Treaty; however, it has not yet passed. China, Russia, and the United States have yet to sign it.
During the 1898 Hague Convention, it was decided that militaries would no longer be able to drop bombs from balloons; however, the practice survived well into World War II. One family in Oregon — including a pregnant woman and her five children — was killed by one of these balloons while picnicking. They are the only people to be killed in the contiguous United States during World War II. The weapon itself is mostly useless as there isn’t enough impact to set the bombs off as they drift from balloons.