The Gemara in Masechet Brachot (63a) relates the following story:

Rav Safra said: Rabbi Abahu recounted, that when Rabbi Chanina the son of Rabbi Yehoshua's brother went down to the diaspora, he determined leap years and new months outside of the land (of Israel). The sages of Eretz Israel sent two Talmidei Chachamim to him, Rabbi Yosi ben Kippar and the grandson of Zecharia ben Kevutal… (When Rabbi Chanina ruled on Halachic questions they began to contradict him): he determined that a thing was impure and they determined that it was pure, he ruled that something was prohibited and they ruled that it was permitted… He said to them: why are you opposing every ruling I make? They said to him: because you are determining leap years and new months in the diaspora… if you will listen (and cease), good, and if not, you will be ostracized and it will be said to our brothers in the diaspora: if you listen, good, and if not, let them go up to the mountain and Achiah (the head of the diaspora Jews) will build an altar, Chananiah (meaning Rabbi Chanina) will play on the harp and all will deny (belief in HaShem) and declare that they have no part in the G-d of Israel. Immediately all the Jews of the diaspora sobbed and cried out: G-d forbid! We have a part in the G-d of Israel! And why is this so severe? Since it says (Yishayahu 2;3): As from Zion will come out Torah and the word of HaShem from Yerushalayim."
 

And so the Rambam rules in Hilchot Kiddush HaChodesh (Ch. 5, Halacha 1 and Halacha 13):

Everything we said regarding determining Rosh Chodesh according to sighting (the new moon), and (adding a month to the year) according to the time or when there is a need, is performed only by a Sanhedrin in Eretz Israel or a Beit Din of authorized sages in Eretz Israel who received their authority from the Sanhedrin. As it was said to Moshe and Aharon: "This month is to you the first of months" and they learned from HaShem, and this was passed down from generation to generation since Moshe Rabbenu, and this is the meaning of it: This testimony will be passed to you and all who will stand in your place afterwards. But at a time when we do not have a Sanhedrin in Eretz Israel, months and leap years are not determined except by the calculation we utilize today.
 

This which we do now, that everyone calculates in his city and announces which day will be Rosh Chodesh and which day will be Yom Tov – we do not determine by our calculation, and we do not rely upon it. As we do not determine leap years and months in the diaspora. And we rely only upon on the calculation and determination of B'nei Eretz Israel. And that which we calculate is only in order to discover it (to make the calculation). Since we know that they rely upon this calculation, we make this calculation in order to know which day B'nei Eretz Israel determined that it (Rosh Chodesh, etc.) will be. And the day determined by B'nei Eretz Israel – this is what will be Rosh Chodesh and Yom Tov, and (the determination) is not because of the calculation we make.
 

And in Sefer HaMitzvot (Positive Commands, 153):

If we were to imagine, in theory, that there would be no Jews in Eretz Israel – G-d forbid that this should happen, since He promised that the signs of the nation will not be completely eradicated – and (additionally) there will be no Beit Din there, and not one in the diaspora which was authorized in Eretz Israel, then our calculation will have no validity whatsoever, since we are forbidden to calculate months and to add months to leap years in the diaspora, but only under the aforementioned conditions, as we clarified: As from Zion will come out Torah.
 

And the Chatam Sofer wrote (Yoreh De'ah 234) regarding the words of the Rambam:
 

It appears from his words that if, G-d forbid, no Jews will remain in Eretz Israel, then even if Jews will dwell in the diaspora, this would be considered the annihilation of the nation, G-d forbid, since we have no semuchin (authorized sages) and we could not establish (new) months and leap years. And according to the Rambam, the fact that the sages of the previous generations determined years and months and sanctified them is not effective unless at least (some) Jews remain in Eretz Israel, even only winegrowers and farmers – that when the appointed time will come, then the winegrowers will determine the times according to the calculations of these predecessors… and if not, the calculation and the sanctification of the previous generations is ineffective, and, G-d forbid, all the Torah will be nullified, and there will be no Israelite nation, and this is (tantamount to) the obliteration of the nation, G-d forbid...
 

In other words, if there are no Jews living in the land of Israel, the result will be the nullification of the Torah and the end of the existence of Am Israel.
 

It is well-known that The Ramban deemed the settlement of the land of Israel a mitzvah from the Torah, whereas the Rambam does not enumerate it among the mitzvot:
 

 (The mitzvah is) that we were commanded to inherit the land which the Lord, may He be blessed and exalted, gave to our fathers - to Avraham, to Itzchak and to Yaakov - and not to abandon it to the hands of any other nation, or to (allow it to) lay in waste. And this is what He said to them: "And you shall clear out the land and dwell in it, for to you I have given the land to possess it."
 

However, the Rambam did rule in several places that there is a mitzvah to dwell in the land, for example: the law which permits one to tell a non-Jew to write a contract on Shabbat to purchase land, even in Syria, because of the mitzvah to settle the land. 

This is also the law allowing a husband to force his wife, or vice versa, to go up (immigrate) to Eretz Israel.
 

Perhaps this can be reconciled, that the Rambam did indeed consider settling the land of Israel a mitzvah of the Torah for all generations, but that he did not include it in the enumeration of the mitzvot because of the following principle which he explained in the Shoreshim (roots):
 

That it is not appropriate to enumerate the commands which embrace the entirety of the Torah. For instance, the Torah includes commands and warnings which do not refer to a specific mitzvah, but encompass them all, in effect saying: do all which I have commanded you to do, and beware of all I have warned you not to do, or do not transgress a thing which I have commanded you. And it is not relevant to count such a command as a mitzvah in and of itself. (Sefer HaMitzvot, Shoresh 4)
 

In light of the aforementioned, we can understand why the mitzvah of dwelling in the land of Israel is considered an all-encompassing mitzvah, since, were the mitzvah of settling the land nullified, then the entire Torah would be nullified, and therefore this mitzvah is the foundation for all the mitzvot.
 

May we merit to do the mitzvah of settling our holy land in its length and breadth, and through this, to achieve the fulfillment of the entire Torah, speedily and in our days.