This verse from Deuteronomy 20:19 outlines a specific rule of warfare: Do not destroy fruit trees when besieging a city. Let's break down the meaning:

"When you shall besiege a city a long time, in making war against it to take it...": This sets the context of a prolonged military siege aimed at capturing a city.

"...you shall not destroy its trees by wielding an ax against them; for you may eat of them.": The primary prohibition is the destruction of fruit-bearing trees. The reason given is practical: the trees provide food that the besieging army itself could use. They are a valuable resource, not an enemy.

"You shall not cut them down; for is the tree of the field man, that it should be besieged by you?": This is the key to understanding the deeper moral principle. The verse poses a rhetorical question: "Is the tree of the field a man, that it should be besieged by you?" In other words, the trees are not combatants. They are innocent and cannot be held responsible for the actions of the city's inhabitants. The verse emphasizes that war should be directed at the human enemy and not at the natural world that sustains life.

Here's a summary of the key meanings and implications:

Practical Conservation: It's a pragmatic rule for preserving a food source.
Humanity in Warfare: It promotes a sense of restraint and respect for life even in the context of war. It limits the scope of destruction to what is strictly necessary to achieve military objectives.
Distinction Between Combatant and Non-Combatant: The tree is not an enemy; the enemy is the people in the city. Therefore, it's immoral to wage war against it.
Environmental Stewardship: Some scholars also see this as an early form of environmental awareness, recognizing the value of trees beyond their immediate utility.

In essence, the verse encourages a measured and ethical approach to warfare, prioritizing human life and avoiding the gratuitous destruction of resources that could benefit both sides in the long run.