[Background: The question was posed whether the oath against "going up as a wall" prohibits only a military invasion of Eretz Yisroel, or even mass immigration with permission from the ruling power. Now the Rebbe will bring proof to the second possibility from the Ramban in his commentary on Chumash.]
In his commentary on Parshas Ki Savo, the Ramban goes a step further. At the end of the Tochacha (the Rebuke, the list of punishments that would come upon the Jews if they violate the Torah), the Ramban writes (on Devarim 28:42):
You must know that the Rebuke, with sicknesses and so on, and all the other punishments, only applies "until He gets you away from the land which you are entering to inherit." But after the Jewish people go into exile, the Torah does not curse them in any way, except that they will have to serve gods of wood and stone. But these punishments only happen when they are in Eretz Yisroel, as we see that the Torah concludes them with the words, 'And you shall be pushed off of the land," which means exile. But after we are in exile, in the land of our enemies, the work of our hands is not cursed...for when we live in exile we have Hashem's promise (Vayikra 26:44), "And even so, when they are in the land of their enemies, I did not reject them nor despise them, to destroy them, to annul my covenant with them, for I am Hashem their G-d."
We see from this that Hashem's promise that there will not be mass destruction, G-d forbid, applies only when we are in the land of our enemies, not in Eretz Yisroel. If so, this decree that we must live in exile and not in Eretz Yisroel is an awesome decree. It is well known that "in every generation they rise up against us to destroy us, but the Holy One, blessed is He, saves us from their hands." But if, G-d forbid, Hashem's promise to save us is not in effect, there is no danger more tremendous and terrifying than that.
If so, we cannot say that Jews may go up en masse to Eretz Yisroel with permission from the ruling power, for even if the government gives permission, the decree of exile - to be in the lands of our enemies - is not being fulfilled.
However, there is nothing wrong with a small portion of the Jewish people living in Eretz Yisroel. As long as the majority of the Jewish people is in the land of their enemies, bearing the decree of exile, Hashem's promise to protect the entire Jewish people from destruction - even those living in Eretz Yisroel - is in effect. The Ramban only means that there will be danger if the majority of the Jewish people ascend as a wall against the will of Hashem, who made the decree of exile. So it comes out the fact that most of the Jewish people lives in exile, in accordance with Hashem's decree, is what saves even the Jews of Eretz Yisroel.