Aliyah to the Land of Israel is the clear mitzvah we find in this week’s parashah, Lech Lecha. Regarding the applicability of this mitzvah in our times, there are several opinions. There is the opinion of Nachmanides, the Ramban, who holds that even in this time—during the exile—there is a positive commandment to make aliyah to the Land, conquer it, and settle it. This is derived from several explicit verses in the Torah.
The most difficult opinion to fully comprehend is Rambam’s. On the one hand he brings all the virtues and praises that the sages state regarding those who live in the Land of Israel, including all the halachot that are derived from them. On the other hand, he does not write that it is a positive commandment from the Torah. For this reason, most of the later halachic authorities (the Acharonim) believe that, according to Rambam, in our time, living in the Land of Israel is a rabbinic commandment—a lofty and exalted one, but nonetheless rabbinic.
We know how much Chasidut cherishes and upholds the principle that, “the words of the scribes [the rabbis] are more endeared than the words of the Torah,” and thus, to a certain degree, one must be even more careful with a rabbinic mitzvah more than a mitzvah from the Torah. Among the later authorities, the one who primarily discusses this topic, and upon whom most of the later poskim rely, is the author of Pe’at HaShulchan, Rabbi Yisrael of Shklov, the prominent disciple of the Vilna Gaon (the Gra). He writes that according to Rambam, this mitzvah is of rabbinic origin. There is another way to explain this, which will be elaborated on briefly later.
In any case, this is the straightforward understanding: Ramban holds that dwelling in the Land of Israel is a mitzvah from the Torah even in our time. Of course, this mitzvah applies to the individual—that's the “catch,” as it were. It is not a communal mitzvah. If it were a communal mitzvah, meaning that everyone must make aliyah, it would be considered an “ascent by force” (עֲלִיָּה בַּחוֹמָה), which is prohibited by one of the well-known Three Oaths, complicating the entire issue.
Rambam does not bring up the topic of the Three Oaths in his Mishneh Torah, including the oath not to ascend by force. He does not mention it in his halachic works, but in his Epistle to Yemen, he mentions is forcefully and states that an ascent by force of the Jewish people to the Land poses a great danger to the people.
In my humble opinion, this might explain why he does not include settling the Land of Israel as a mitzvah from the Torah in the Mishneh Torah, and also why he does not mention the oath in his legal writings. There is something in the oath, “I make you vow, O daughters of Jerusalem” that nullifies the mitzvah, so to speak, because how could there be a mitzvah that does not apply to everyone together? Logically, a mitzvah should apply and obligate all, but here, it does not apply to everyone—only to those who, in a special way, desire (the essence and meaning of the Land of Israel, אֶרֶץ, is to desire, to want, רָצוֹן) it —to those who truly “pray through their land.”
The author of Pe’at HaShulchan brings all the early authorities (such as the Tashbetz, Rashbash, and others) who agree with Ramban’s view and states, “this is the primary opinion.” In other words, the one in our generation who rules that in practice the halachah follows Ramban is the Pe’at HaShulchan. It also seems that the Minchat Chinuch, who came after him and is one of his admirers, as well as many later authorities, follow his view, accepting Ramban’s opinion as primary—that the mitzvah of settling the Land of Israel is a positive commandment from the Torah, even in our time.
Again, Rambam does not list it as a positive commandment, although he does bring in his halachic writings all the virtues and praises of dwelling in the Land of Israel—everything that the sages said at the end of tractate Ketubot in the discussion on the Mishnah that states “All are brought up to the Land of Israel” (מַעֲלִין לְאֶרֶץ יִשְׂרָאֵל).
