White phosphorous is often used as an incendiary weapon, but  it serves well enough as a lethal purely chemical burn agent by creating phosphoric acid in the skin, and the smoke is used as a primarily nonlethal chemical weapon at high concentrations - it's used in Iraq to flush out tunnels.

They had these aircraft before 1918.

If Phosphorous isn't bad enough, mustard gas was also used on the Kurds by the RAF during the 20's - 30's.