The Xenos have overrun Trebulon 5 and 6. The Colonial Marine Headquarters on Trebulon 7, heavily fortified and defended by a brigade of veterans, has been designated as a final fallback position before the Trebulon system is abandoned. You are expected to give all and sacrifice everything - even your life if need be - to make a last desperate stand that will prevent the Xenos from spreading to other systems. The yellow bar next to your selected weapon indicates your ammunition level. If you have no more ammo left, you can use your chain saw to cut your way through.