Answer

Read Nachmanides’ own words. He explains everything:
The fourth commandment with which we were commanded is to inherit the land that God, blessed and exalted, gave to our forefathers—to Abraham, to Isaac, and to Jacob—and not leave it in the hands of any other nation or desolate. This is what He said to them (Masei 33, and Nachmanides there): “You shall dispossess the land and dwell in it, for to you I have given the land to possess it, and you shall inherit the land.” This same idea is repeated regarding this commandment in other places, as He said, may He be blessed (Deuteronomy 1), “Come and possess the land that I swore to your fathers,” and He spelled it out for them in this entire commandment with its borders and boundaries, as He said, “Come to the hill country of the Amorites and to all their neighbors in the Arabah, in the hill country, and in the lowland, and in the Negev, and by the seacoast,” etc.—that they should not leave any part of it. The proof that this is a commandment is from what He said, may He be blessed, regarding the spies (ibid.): “Go up and take possession, as the Lord your God has spoken to you; do not fear and do not be dismayed.” And He also said (Deuteronomy 9), “When the Lord sent you from Kadesh-barnea, saying: Go up and possess the land that I have given you,” and when they were unwilling to go up in response to this statement, it is written, “You rebelled against the word of the Lord your God, and you did not believe Him and did not listen to His voice”—which indicates that this was a commandment, not merely a promise and assurance. This is what the sages call (Mishnah Sotah 8:7) an obligatory war. And so they said at the end of tractate Sotah (44b): Rav Yehudah said: Joshua’s war of conquest is, according to all opinions, obligatory; David’s war for expansion is, according to all opinions, optional. And the wording of the Sifrei (Judges 17:14) is: “And you shall inherit it and dwell in it” — by the merit of inheriting, you will dwell. And do not make the mistake of saying that this commandment is the commandment concerning the war against the seven nations, whom we were commanded to destroy, as it says (end of parashat Shoftim), “You shall utterly destroy them.” That is not so. For we were commanded to kill those nations when they wage war against us; but if they wished to make peace, we would make peace with them and leave them under certain known conditions. But the land itself we may not leave in their hands, nor in the hands of any other nation, in any generation. Likewise, if those nations fled from before us and went away, as they said (Deuteronomy Rabbah, Shoftim), “The Girgashite turned away and left,” and the Holy One, blessed be He, gave them a good land like theirs—this is Africa—we are commanded to come into the land, conquer the provinces, and settle our tribes in it. Likewise, after we cut off the nations mentioned, if our tribes later wished to leave the land and conquer for themselves the land of Shinar, or the land of Assyria, or other places, they are not permitted to do so, for we were commanded regarding its conquest and settlement. And from their statement, “Joshua’s war of conquest,” you can understand that this commandment concerns conquest. And so they also said in the Sifrei (end of Eikev): “Every place on which the sole of your foot treads, I have given to you…” He said to them: every place that you conquer outside these places shall be yours. Or perhaps they had permission to conquer lands outside the Land of Israel before conquering the Land of Israel? Therefore the verse says: “And you shall dispossess nations greater and mightier than you,” and only afterward, “Every place…” And they said: And if you ask, why did David conquer Aram Naharaim and Aram Tzovah when the commandments do not apply there? They said: David acted contrary to the Torah. The Torah said: after you conquer the Land of Israel, then you may conquer outside the land—and he did not do so. Thus we have been commanded regarding conquest in every generation. And I say that the commandment the sages so greatly emphasized—namely dwelling in the Land of Israel—so much so that they said (Torat Kohanim Behar 5:4, and similarly Ketubot 110b and elsewhere, and Hilkhot Melakhim end of ch. 5) that anyone who leaves it and lives outside the land should be regarded in your eyes as one who worships idols, as it says, “For they have driven me out today from joining the inheritance of the Lord, saying: Go, serve other gods”—and many other strong statements they made about it, all of this stems from this positive commandment with which we were commanded to inherit the land and dwell in it. If so, it is a positive commandment for all generations, binding on every individual among us, even in a time of exile, as is known from the Talmud in many places. And the wording of the Sifrei (Re’eh 12:29) is: It once happened that Rabbi Yehudah ben Beteira, and Rabbi Matya ben Heresh, and Rabbi Hananiah the nephew of Rabbi Yehoshua, and Rabbi Natan were leaving for outside the land, and when they reached Paltia they remembered the Land of Israel, raised their eyes, tears streamed from them, they tore their garments, and they read this verse: “And you shall inherit it and dwell in it and observe to do [all that is written there].” They said: Dwelling in the Land of Israel is equal to all the commandments in the Torah.