Why did Hashem command to ruin their horses and burn their chariots?
Radak: Nochrim trust in their chariots. They do not know that "Sheker ha'Sus li'Sshu'ah" (Tehilim 33:17). Hashem did not want Yisrael to take them 1 , lest they rely on them in their wars. Malbim (6) - the king is commanded not to amass too many horses (Devarim 17:16)!
Radak: The previous wars were without horses and chariots. They were not commanded about other animals; only horses are used for war. They were not Asur b'Hana'ah. One may not kill a living being without need; it suffices to cut off their feet. (Seemingly, even if only one foot is cut off, the horse is not useful for war!