(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. After they conquer outside of Eretz Yisrael, whence is it derived that mitzvoth obtain there?
It follows thus: It is written here (in respect to the land outside of Eretz Yisrael proper) "shall be" (viz. "shall be your border). Just as there, mitzvoth obtain, so, here, mitzvoth obtain. And if you would ask: How is it that David conquered Aram Naharayim and Aram Tzovah and mitzvoth do not obtain there? They said: David did not act in accordance with the Torah.
The Torah said: After you conquer Eretz Yisrael, you are permitted to conquer outside of it. He did not do so, but he turned back and conquered Aram Naharayim and Aram Tzovah, not having driven out the Yevussi, who were close to Jerusalem. The Holy One Blessed be He said to him: If you did not drive out those close to your palace, how could you turn back and conquer Aram Naharayim and Aram Tzovah!