Because you know that you will never advance again, you begin to attempt to make it impossible for your enemy to use what you are giving up. You burn all fields, destroy all bridges, and dismantle railroad lines and roads. Finally, you poison the water so that nothing can survive in the place you have left to the enemy.
In the end, you have destroyed the earth. Nothing will grow there. Nothing will graze there. The barren earth is the only result of your actions.
And you huddle with what's left of your allies in the dark and wait for the end.
I've often wondered if, in their last desperate stand, the people who have done this realize that they've effectively destroyed themselves, taken away all their hope, and finally ended with absolutely nothing to show for it except empty stomachs and vile hatred. Do they realize it? Or does the hatred feed them, give them strength? Do they believe that they are the only ones who are right? Do they believe they have saved themselves?
I don't know. All I know is that it must be dark and cold to huddle alone, wondering if it's going to work, and whether you've destroyed yourself in the process. The nagging feeling that you did must be a terrible thing to bear on your conscience.
See, that military history degree of mine makes for interesting thoughts. :)