From Halopedia, the Halo wiki
War is an achievement in Halo: The Master Chief Collection. It is awarded for activating the first Halo 4 Terminal on Reqiuem. The achievement is worth 5 Gamerscore and can be unlocked on any difficulty.
After entering the terminus tower the player will enter a room where the structures hanging from the ceiling will retract into the walls on the side. At the far end of the room is a staircase that leads to the elevator which is the objective. The terminal is located under the staircase.