רמב"ן — והורשתם את הארץ וישבתם בה / כי לכם נתתי את הארץ לרשת אתה

Q — 'והורשתם את הארץ וישבתם בה.' Rashi: a conditional — if you dispossess the inhabitants, you'll succeed in dwelling there; otherwise not. Just practical guidance.

A — Ramban famously reads this as a mitzvat aseh — the foundational command to dispossess and dwell in Eretz Yisrael, which he counts in his Hasagot on Sefer HaMitzvot as one of the obligations Rambam omitted. 'לא ימאסו בנחלת ה'.' Even after the original conquest, Israel could not abandon the land for Shinar, Ashur, or anywhere else without violating this mitzvah. He distinguishes it sharply from the milchemet sheva amamim — even if those nations flee or surrender (per the famous derasha about the Girgashi who went to Africa), the obligation to settle the land remains. This is the source, on Ramban's read, for Chazal's many statements: that one who leaves Eretz Yisrael is 'כעובד עבודה זרה,' that yishuv ha'aretz is 'שקולה כנגד כל המצות,' and the moving Sifrei episode of the rabbis tearing their clothes when leaving Eretz Yisrael.