The war has been going on for months, and seem like it will never end.
They had to protect others and themselves, but they also had to try not to hurt the crystal ponies badly. So all the soldiers are exhausted.
The background story of this scene is that crystal army captured Fleetfoot (I haven’t think of the reason) and the rest of them saved her but also got hurt very badly. After they got away from the crystal army, they arrived at a destroyed village.