מדרש על יהושע 1:4

(Devarim 11:24) "Every place where the soles of your feet tread will be yours": If (the purpose of this is) to teach about the boundaries of Eretz Yisrael, this is already written, viz. (Ibid.) "From the desert and the Levanon, from the river, the river Yerath, until the western sea shall be your border." What, then, is the intent of "where the soles of your feet tread"? He told them: Every place that you conquer, aside from these (specified) places shall be yours. — But perhaps this gives them license to conquer outside Eretz Yisrael before they have conquered Eretz Yisrael proper! It is, therefore, (first) written "and you will inherit nations greater and mightier than you," and (only) afterwards "Every place where the soles of your feet tread shall be yours" — that Eretz Yisrael not be polluted with their idolatries while you are (busy) conquering outside of it. But, after you conquer Eretz Yisrael, you are permitted to conquer outside of it.