We commonly say that this verse prohibits "wanton destruction", and indeed the Sefer HaChinukh explains that "gratuitous" (levatala) destruction is forbidden, but in practice any use of something which is wasteful, even if it is not actually destructive, is forbidden. We see this from the Torah itself. After all, the cutting of the fruit trees is not gratuitious but rather for the purpose of the siege: either to build fortifications or (according to some Rishonim) to demoralize the enemy. And non-fruit trees may indeed be cut down for this purpose, as the following verse explains. But for a fruit tree, which provides us with food, this would be wasteful.
Here are some examples: The gemara implies that it is wasteful to consume expensive foods when cheaper ones are just as good and, most importantly, just as healthy (Shabbat 140b). It is forbidden to tear the cloth of a garment to test someone's control of his anger if tearing the seams will serve the same purpose. (Kiddushin 32a.) It is forbidden to cut down a fruit tree for its wood if the tree is worth more for its fruit. (Bava Kamma 91b.)
We find a very interesting parallel to this idea in the required treatment of fruits of the shemitta year. The Torah tells us that shemitta produce is "to eat" (Vayikra 25:6), and our Sages inferred, "to eat - and not to destroy". (Pesachim 52b.) However, not only outright destruction is forbidden, but also any use which falls short of each food's most proper and elevated use. For instance, food which is normally eaten cooked should not be eaten raw, and vice versa. (Rambam Shemita veYovel 5:3.)
Indeed, the Rambam writes that the very same mishna teaches the prohibition to cut down fruit trees in the shemitta year, because of the kedusha of the fruits, and the prohibition to cut down fruit trees in any year, because of bal tashchit (Commentary on the Mishna Sheviit 4:10.) And regarding both transgressions the Ramban, disputing the Rambam, counts a positive mitzva to eat the fruit in the proper, as opposed to improper, way (List of omitted positive mitzvot #3 and #6).
MANIFEST AND LATENT HOLINESS
The special rules regarding shemitta produce are a result of its unique holiness, kedushat sheviit. This in turn is an expression of its special status as a gift of HaShem to man and beast alike, a gift which may be enjoyed by all but appropriated by none (Vayikra 25:6-7). The impressive parallel to the rules of bal tashchit suggest that at a lesser level all useful objects have a certain latent sanctity, as they too were given to us by HaShem to enjoy, and not to destroy. In the sabbatical year, all produce is completely ownerless, but even in normal times our private ownership is limited, and not absolute.
A MORAL MESSAGE
We may ask, since the principle of bal tashchit extends to all useful objects, why does the Torah present this mitzva specifically in the context of fruit trees in wartime?
Fruit trees in the Torah represent our unique human status. At the time of the creation, fruit trees were designated for specifically human consumption, unlike vegetables which were given to animals as well (Bereshit 1:29-30). When the gemara infers from the unusual wording of our opening passage that a human being is in fact comparable to a tree of the field, it likens a fruit tree to a worthy Torah scholar (Taanit 7a), thus identifying Torah insight as the highest level of human development.
War, on the other hand, exemplifies a negative, destructive activity. Although we are not pacifists and do recognize the need for war, Judaism does not glorify war but rather views it as an unfortunate necessity. Armaments are not considered an ornament or garment on Shabbat, since they will be superfluous in the ideal world which will exist at the time of the Redemption. (Shabbat 63a.)
While both constructive and destructive activities are necessary in life, there is a definite distinction between them. Our highest faculties, our spiritual "fruit trees", should be devoted to positive, fruitiful activities; whereas battling and containing evil, while important, should be left to our baser faculties. Our unique human spiritual capacity should not be wasted in petty battles against passing evils, but rather should be devoted to increasing goodness and in particular to the study of Torah.
Rabbi Meir's book Meaning in Mitzvot is now available as an e-book through Amazon, ibooks, Google Play and B&N.