‘ And Joshua did to them as YHWH commanded him, he houghed their horses and burned their chariots with fire.'
Joshua did not question YHWH's command. He obeyed. It must have been tempting to keep the horses, especially with the chariots. But YHWH had commanded otherwise. We notice that they were not said to be ‘chariots of iron'. There were no iron accoutrements on these chariots. Those would come later. We must presume that the houghing of the horses did not render them totally useless or why keep them alive at all? It meant doing something that prevented them from performing at their best, and being usable for military purposes. Alternately the purpose may have been to keep them for breeding purposes, but that would contradict God's earlier command.