When you lay siege to a city for an extended time while fighting against it to capture it, you must not destroy its trees by putting an axe to them, because you can eat their fruit. You must not cut them down. Are the trees of the field human, that you should besiege them?
19. When thou shalt besiege a city a long time. I have not hesitated to annex this precept to the Eighth Commandment, for when God lays a restraint on the liberty of inflicting injuries in the very heat of war, with respect to felling trees, much more did He desire His people to abstain from all mischievous acts in time of peace.
They are here directed what method to take in dealing with the cities (these only are mentioned, Deu 20:10, but doubtless the armies in the field, and the nations they had occasion to deal with, are likewise intended) upon which they made war.
Commenting on Deuteronomy 20:10-20
When thou shalt besiege a city a long time, in making war against it to take it,.... Before it will surrender; it holding out the siege a considerable time: the Hebrew text says, "many days" (c); which the Targum of Jonathan interprets of all the seven days, to make war against it, in order to subdue it on the sabbath day.