NOTES for Num 31:1-54
The war described in the Book of Numbers seems astonishingly cruel to today's reader. Even the Torah commands that civilians not be destroyed during war and that everything around not be demolished without need. True, there it speaks of war in general. An exception is made for the land that God had appointed for His people. It had to be cleansed of pagans.
This does not necessarily mean total extermination; expulsion was also possible. The main problem was pagan altars, which were not to remain in the land. Together with the altars, those to whom they belonged inevitably had to disappear as well. In those times every nation and tribe had its own god and its own sanctuaries, without which neither nation nor tribe could exist. Peoples disappeared together with the altars of their gods, and the gods passed into oblivion together with the peoples who worshiped them.
In this case, however, everything was somewhat more serious. In addition to public and communal altars and cults, the ancients also had household gods, worshiped at household altars. These were mainly patrons of the clan, family, and house, and figurines of such household gods often traveled with those who worshiped them. Without them, it was impossible to move to a new place: it would remain without blessing, and the new house would not receive those moving in. A girl who married and moved into her husband's house was obliged to honor the gods of her husband and of her new home, yet no one could prevent her from taking her own household gods with her. This was even directly necessary: leaving the sacred figurines at home meant breaking with her father's house, which could have legal consequences, for example the loss of inheritance rights. Therefore, in mixed marriages, images and figurines of local pagan gods worshiped by pagan wives would inevitably appear in Jewish tents. This is why it is said of them that they lead God's people into sin, and this is also why such ruthlessness is shown toward the women.
Of course, this is not a real solution to the problem; paganism is always rooted in the soul. It is rather an attempt to solve the question in the barbarically simple way people in those places solved it at that time. Nevertheless, the problem was truly serious and demanded a solution.