Control the middle!

Control the middle! was mostly said during Valhalla matches in Halo 3.

The map has a middle ridge area separating the two halves of the map. If your troops were able to control the middle area then you were probably winning as you could shoot the members of the other team as they spawned or as they came to try and assault your half of the map. Hence the verbal command to, "Control the middle!", which was probably what everyone had been thinking anyway.

Control the middle! does apply to a lot of FPS maps, and it gets repeated from time to time in other circumstances as well.