The Ramban holds that permission granted by a gentile government for Jews to return to Eretz Yisroel does not nullify the oaths. In his Sefer Hageulah, end of Shaar 1 (p. 274 in the Chavel edition), he asks: Why were the Jews at the time of the Purim miracle still scattered in all the states of King Achashverosh? Just a few years earlier, the first Persian king Cyrus had given permission for the Jews to return to Eretz Yisroel and build the Temple (Ezra 1:3). The permission to build the Temple was later revoked, but we should still expect to find most of the Jews back in Eretz Yisroel. Yet the vast majority remained in exile (and only 42,360 Jews were back in Eretz Yisroel, according to Ezra 2:64). And even later, when Darius reinstated the permission, only about 1500 Jews came up with Ezra from Babylonia (Ezra 8:1-20). The answer is, he says, that the Jews would not have taken advantage of these kings’ offers had they not been foretold by a prophet, speaking in the name of Hashem. That prophet was Yirmiyahu, who said (29:10), “When seventy years of Babylonia are complete, I will revisit you.” Now, the Jews were uncertain whether these kings had meant to give permission for all the tribes of Israel to return, or only for Yehuda. And even if they had meant to give permission to all of Israel, perhaps Yirmiyahu’s prophecy had only referred to those Jews living in Babylonia proper, not in all the 127 Persian states. The king’s permission was not enough; they needed Hashem’s permission as well. Without Hashem’s permission, they had no right to leave exile; this would be “forcing the End.”
We see clearly that the Ramban did not agree with the contention that permission from a king alone is proof of a Divine visitation. For the Jews in Ezra's time, nothing short of prophecy was enough to warrant their return, and they were not sure if the prophecy referred to all of them.
The Ramban at the beginning of Vayishlach writes: “This passage (about Yaakov meeting Esav) is a guideline for all generations, because everything that happened to Yaakov Avinu with his brother Esav happens to us continually with Esav's descendants. We must therefore adopt the righteous Yaakov's approach, to prepare ourselves in the three ways that he prepared himself: tefillah, gifts, and saving by way of war, to flee to safety.”
Lest anyone think that the Ramban means that war and fleeing to safety are both valid options during exile, the Chofetz Chaim writes (on Parshas Devarim): “The Torah teaches us not to resist the nations even when they fight against us. We must follow in the footsteps of Yaakov Avinu in his encounter with his brother Esav. As the Ramban writes in Vayishlach, everything that happened between Yaakov and Esav happens to us constantly with Esav’s children…” and he continues to quote the Ramban. It is clear from the Chofetz Chaim that he understood that “to flee to safety” is the Ramban’s adaptation of “war” for our times.
“As long as we walked on that well-tread path, Hakadosh Baruch Hu saved us from their hands. But since we have strayed from the path and new leaders have arisen who chose new methods, leaving behind our ancestors’ weapons and adopting the methods of our enemies, we have fared worse and worse, and great travails have befallen us. May Hashem have mercy on our people and restore our judges as of old,” concludes the Chofetz Chaim.
Alternatively, it is possible that the Ramban meant real war, but only in reference to Yaakov himself. The word “war” refers back to the “three ways that he (Yaakov) prepared himself” and not to us in later generations.
This seems to be how the Abarbanel understood it. In his commentary on Vayishlach he echoes the Ramban: “Just as Yaakov prepared himself with prayer, gifts and war, so it will happen to us in all generations, that our efforts to be saved from Esav and his descendents will be, firstly, by prayer and supplication before the G-d of Yaakov, with gifts, bribes and presents to him, and with war – to flee and save from his hand.”
The Ramban in Sefer Hamitzvos writes, "We were commanded to take possession of the land that G-d promised to our forefathers, to Avraham, to Yitzchak and to Yaakov, and that we should not leave it in the hands of any other people, or leave it desolate... We must not leave the land in their [the Canaanites] hands or in the hands of any other people in any generation." Many have asked: how could the Ramban say that we must conquer Eretz Yisroel in every generation, if there are these three oaths prohibiting such a conquest? Obviously, these people say, the Ramban must hold that these oaths are not halachically binding.
However, one cannot understand the Ramban to mean we must conquer Eretz Yisroel in every generation, including during exile, because if one takes a look further in the Ramban one will see that it's not so. He brings a proof from Chazal's statement that Dovid Hamelech was wrong to conquer Syria before completing the conquest of Eretz Yisroel, and he ends off, "So we see that we were commanded to conquer it in all generations."
Then he says, "And I say that the mitzvah of which Chazal speak highly, living in Eretz Yisroel...is all part of this positive commandment, for we were commanded to take possession of the land in order to live in it. If so, it is a positive commandment for all generations, in which each one of us is obligated, even during exile."
We see clearly that the Ramban needed a second proof, from the fact that Chazal speak highly of living in Eretz Yisroel, that the mitzvah applies during exile. His first proof from Dovid Hamelech did not cover exile.
Why then does he say "we were commanded to conquer it in all generations"? The answer is that he is anticipating someone defending the Rambam, who does not count this mitzvah, by saying that the mitzvah was a one-time-only command to Yehoshua to conquer the land. One-time-only mitzvos aren't counted in the 613, as per the Rambam's third rule of counting mitzvos. The fact that the mitzvah applied to Dovid Hamelech proves that it was not a one-time-only mitzvah. To use the halachic terms, it is ledoros (for the generations) and not leshaah (one-time-only). And that is exactly what the Ramban means when he says we were commanded to conquer it in all generations - that it is a permanent mitzvah. But there are certainly times when the mitzvah of conquest is suspended, namely during exile. In this respect it is just like all the mitzvos relating to the Beis Hamikdash and the korbanos, which are considered permanent mitzvos, counted among the 613, yet are suspended during exile.
It is true that in the end, the Ramban proves that the mitzvah of living there applies during exile too, but that is only the mitzvah of living there, not the mitzvah of conquering. You have to read the Ramban carefully. Up until this point, the Ramban calls the mitzvah lareshes, "to take possession" of the land. Now he makes an additional point: that the mitzvah of living in Eretz Yisroel of which Chazal speak is also part of this same mitzvah, "for we were commanded to take possession of the land in order to live in it." In other words, the real mitzvah is to live in the land, and conquering is only a hechsher mitzvah - a preparatory stage in order to reach the mitzvah. Writing tefillin is preparation for putting them on, but the mitzvah is only to put them on. If one has pre-written tefillin, he is under no obligation to write them. Building a succah is preparation for sitting in it, but the mitzvah is only to sit in it. If one has a pre-built succah he does not have to build another one. Similarly, from the fact that Chazal speak highly of living in Eretz Yisroel even during exile, the Ramban concludes that the real mitzvah is living there, not conquering it. Conquering in the time of Yehoshua and Dovid was only a preparation that made it possible to live there, but if one can live there without conquering it, he also fulfills the mitzvah. Therefore, even during exile when conquest is forbidden under the oaths, it is possible to live there and fulfill the mitzvah.
And on the contrary, the Ramban actually sounds like he makes the unstated assumption that there is an oath that forbids conquest during exile. Otherwise, why does he have to bring another proof that the mitzvah applies during exile? What should be the difference between Dovid Hamelech's time and our time, if not the oath?
And the Ramban expresses no surprise that Chazal in their time speak only of the mitzvah of living in Eretz Yisroel, not conquering it. He merely says that despite the suspension of the hechsher mitzvah of conquering, it is still possible to do the mitzvah itself, living there.
Also note his words: "It is a positive commandment for all generations, in which each one of us is obligated, even during exile." Why does he say "each one of us"? Because he knows that if the mitzvah were on the Jewish people as a whole, it would be impossible to fulfill it during exile without conquest. There is no way that any power ruling the land would allow the entire Jewish people to return to the land en masse - they would see it as a threat to their rule. They would only allow one Jew here and one Jew there to come. Therefore, the Ramban says, it is a mitzvah that whatever individual Jews can come and live there, should do so.
Furthermore, pay close attention to the Ramban's words: "And I say that the mitzvah of which Chazal speak highly, living in Eretz Yisroel, to the point that they said in Kesubos: Anyone who goes out of it and lives in Chutz Laaretz should be in your eyes as if he worships idols..." If you look in Kesubos 110b, the Gemara doesn't say "anyone who goes out of it." It says, "Anyone who lives in Chutz Laaretz is similar to one who has no G-d... is as if he worshipped idols." Why did the Ramban misquote the Gemara?
The answer is that there are really two different statements of Chazal: one in the Gemara, and one in the Toras Kohanim on Parshas Behar (Vayikra 25:38). The Torah says, "To give you the Land of Canaan, to be your G-d." Chazal comment, "Every Jew who lives in Eretz Yisroel accepts upon himself the kingdom of Heaven. And whoever goes out to Chutz Laaretz is as if he worships idols." Rashi quotes this in his commentary on Vayikra 25:38. According to this statement, only one who is born in Eretz Yisroel and leaves it is considered as if he worshipped idols, but one who is born in Chutz Laaretz is under no obligation to move to Eretz Yisroel.
It is this statement that the Ramban quotes. Based on this, it seems clear that the Ramban did not even mean that an individual who is able to move to Eretz Yisroel should do so. He meant that it is an optional mitzvah that one fulfills if he lives there, but is not obligated to go.
The Gemara in Kesubos indeed says, "Anyone who lives in Chutz Laaretz..." It sounds like even those born in Chutz Laaretz are not allowed to stay there. This could be understood in the context of the Tosefta Avodah Zarah 5:2, which says, "Rabbi Shimon ben Elazar says: Jews in Chutz Laaretz worship idols in purity. How so? If an idol worshipper makes a feast for his son, and goes and invites all the Jews in his city, even if they eat their own food and drink their own drinks and their own waiter waits on them, they are idol worshippers." Here too, the Gemara in Kesubos is talking about Jews who socialize with the gentiles, which has bad consequences even for those who are born in Chutz Laaretz.