The Ramban in Sefer Hamitzvos writes explicitly, "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." Now, 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 the Ramban must hold that these oaths are merely Agadta, not halachically binding.
Looking forward to your answer,
Do you really think the Ramban means we must conquer Eretz Yisroel in every generation, including during exile? Take a look further in the Ramban and you'll 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.
So why does he say "we were commanded to conquer it in all generations"? 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 they are suspended during exile.
In the end, the Ramban does prove that it's a mitzvah during exile too. But let's be exact: he proves that the mitzvah of living there applies during exile, 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.
This fits very well with the words of the Ramban. 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 the Torah. 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 means to quote. 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.
Although the Ramban says he is quoting Kesubos, perhaps he meant that the Gemara in Kesubos is to be understood like the Toras Kohanim, as only applying to one who is born in Eretz Yisroel and leaves. Or, it could be that the Ramban was inexact and really meant to quote the Toras Kohanim.
The Gemara in Kesubos, which seems to say that even those born in Chutz Laaretz are not allowed to stay there, 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 went and invited 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.
Now, you may ask: is this how the poskim really understood the Ramban? The answer is certainly. Rabbi Moshe Feinstein, the foremost posek of our time, writes as follows: "Regarding your question if there is a mitzvah nowadays to live in Eretz Yisroel, as the Ramban says, or if there is no mitzvah nowadays, as Rabbeinu Chaim says, quoted in Tosafos on Kesubos 110b: Most poskim hold it is a mitzvah. But it is obvious that this is not an obligatory mitzvah in our time, for if so, it would result in a prohibition to live in Chutz Laaretz because doing so would be violating a positive commandment, like wearing a four-cornered garment without tzitzis, which is forbidden because it transgresses the positive commandment of tzitzis. But we only find a prohibition regarding one who lives in Eretz Yisroel, who is forbidden to leave with the intention of dwelling permanently in Chutz Laaretz - see Rambam Hilchos Melachim 5:9. And this prohibition is certainly not a negative commandment. Had it applied to the Jews of Chutz Laaretz, the Rambam would have written, 'It is forbidden to live in Chutz Laaretz unless there is a strong famine in Eretz Yisroel.' So we see that it is a Rabbinic prohibition, only for those who dwell in Eretz Yisroel. But as far as the Biblical positive commandment, it is not obligatory, only that if one lives there he fulfills the mitzvah. And in my Chiddushim I have written at great length about the words of Rabbeinu Chaim in Tosafos in Kesubos. Now, since it is not an obligatory mitzvah, you must definitely take into consideration the concern of Rabbeinu Chaim in Tosafos as to whether you will be able to keep the special mitzvos of Eretz Yisroel." (Igros Moshe, Even Hoezer 1:102)
In short, Reb Moshe says that even the Ramban only holds that there is an optional mitzvah nowadays to live in Eretz Yisroel. What makes it optional, if originally it was obligatory? Clearly the oaths. It can't be obligatory, because then all Jews would have to go and they would transgress the oaths.
And this is how the Ramban has been understood for hundreds of years. Rabbi Shlomo ben Shimon Duran, the Rashbash (1400-1467) wrote: "There is no doubt that living in Eretz Yisroel is a great mitzvah at all times, both during and after the time of the Temple, and my ancestor the Ramban (he was a sixth generation descendent of the Ramban) counted it as one of the mitzvos, as it says, 'You shall take possession of it and live in it,' and so is the opinion of my father the Rashbatz in his work Zohar Harakia. And even according to the Rambam who did not count it as a mitzvah, it is at least a Rabbinic mitzvah, besides the many other benefits of living there. However, during exile this is not a general mitzvah for all Jews, but on the contrary it is forbidden, as the Gemara says in the last chapter of Kesubos, that this is one of the oaths that the Holy One, blessed is He, made the Jews swear: that they not hurry the end and not go up as a wall. Go and see what happened to the children of Ephraim when they hurried the end! However, it is a mitzvah for any individual to go up and live there, but if there are considerations that prevent him he is not obligated." (Shailos Uteshuvos Rashbash, siman 2)
Similarly, Rabbi Shmuel Salant quotes the law that a wife may force her husband to move to Eretz Yisroel (Kesubos 110b), and then asks why – even according to the Ramban, he says, there is no obligation on every Jew to move to Eretz Yisroel, since this is one of the Three Oaths. He therefore explains that she can only force him to move if she is willing to move even without him. In that case, if he refuses to come along, he is not fulfilling his marital obligations to her, and he must divorce her. But if she wants to move only with him, then he has no obligation to move. In other words, the reason why he must divorce her if she is willing to move to Eretz Yisroel is not because his refusal to move to Eretz Yisroel is considered neglecting his obligations as a Jew. It is only because once she moves, he would be neglecting his obligations as a husband. (Printed in Tzefunos, year 3 issue 1, p. 46)
Rabbi Yehoshua of Kutna also understood that the Ramban would not obligate conquest nowadays. He says that since the full mitzvah is conquering and we don’t do that today, we aren’t really doing the mitzvah when we live in Eretz Yisroel. What we are doing is preparation for the mitzvah, just like baking matza is a preparation for the mitzvah of eating matzah. (Yeshuos Malko, Yoreh Deah 66)
It has also been proposed by Rabbi David Smith (Derech Hachaim, p. 25) that the Ramban's intent was that even the commandment that a Jew should live in the land as an individual during exile applies only when living in the land is consistent with exile, that is, when a non-Jewish government rules the land. But living under a Jewish government such as the State of Israel might itself constitute a violation of the oath. The Ramban felt no need to mention this exception to the commandment because he did not foresee the rise of a Jewish government in the Holy Land before moshiach.
It's interesting that according to way we're learning the Ramban, the Megillas Esther's question on him falls away. The Megillas Esther brings Rav Yehuda's opinion in Kesubos 110b that even individual Jews are forbidden to return to Eretz Yisroel during exile, based on the verse, "They will be brought to Bavel and remain there" (Yirmiyah 27:22). If there is a mitzvah to live in Eretz Yisroel even during exile, he asks, then how could another prophet come and contradict the words of Moshe Rabbeinu? According to the way we're learning, however, the original mitzvah was to live in Eretz Yisroel, with conquest as a preparatory step. The Three Oaths forbade this preparatory step, and automatically the mitzvah was reduced to optional status - because if it had obligatory status, it would automatically lead to conquest. This is not considered Shlomo Hamelech overruling a mitzvah of Moshe Rabbeinu, because the Three Oaths are not directed at the mitzvah itself, only at the preparatory step. Now that the mitzvah is only optional, Yirmiyah Hanavi could come and forbid even individuals from going (according to Rav Yehuda) because they were not obligated in any case.
Finally, let me tell you that you cannot learn that the Ramban in Sefer Hamitzvos didn't treat the oaths as a real halachic prohibition, because then you would be faced with a contradiction in the Ramban's own writings. In Sefer Hageulah, end of Shaar 1 (p. 274 in the Chavel edition), he writes that the reason most Jews did not go up at the beginning of the Second Beis Hamikdash is that Jews were uncertain whether King Cyrus had meant to give permission for all the tribes of Israel to return, or only for Yehuda. And even if he had meant to give permission to all of Israel, they did not wish to force the end, for they knew that Yirmiyahu’s prophecy of a 70-year-long exile had only referred to those Jews living in Babylonia proper, not in all the 127 Persian states.