It is earned when one finishes the campaign mission Tsavo Highway on the Normal, Heroic, or Legendary difficulty. On Halo: The Master Chief Collection, you can be able to unlock this achievement on Easy.
The Halo 3 achievement gives 20 gamerscore and is represented by a blue plate with a Roman numeralIII on it.
Note: You must finish the level completely through from Mission Start, otherwise it won't count.