פירוש על עבודה זרה 106:6

The baraita above could have just said that if the idolaters intend to return, it is prohibited. Why mention the war waged by Joshua? The Talmud will now say that the baraita does so in order to teach something else. The issue is a case where an Israelite set up something to be worshipped, but an idolater worshipped it. Although generally a person cannot make something prohibited when it is not his, in this case he does. We will now see that this is derived from the conquest of the land. The baraita teaches us that the laws of idolatry can be learned from what occurred during this period.