Originally Posted by
Sekai
abandoning a valley that you own causes it to respawn all the troops in it immediately. You cannot get a medal from attacking an empty valley else people would just scout spam all day long for them, so conquer/abandon a valley right next to your city is much faster and more efficent than sending out armies to all different valleys that are further and further away.
Edit - not sure what you mean from the question, your own troops will return to your city automatically just the same as if you had recalled them without abandoning.