Deuteronomy 20:20

Overview - Deuteronomy 20
1	The priest's exhortation to encourage the people to battle.
5	The officers' proclamation of who are to be dismissed from the war.
10	How to use the cities that accept or refuse the proclamation of peace.
16	What cities must be devoted.
19	Trees of man's meat must not be destroyed in the siege.

Deuteronomy 20:20  (King James Version)
Only the trees which thou knowest that they be not trees for meat, thou shalt destroy and cut them down; and thou shalt build bulwarks against the city that maketh war with thee, until it be subdued.