There is perhaps no Rashi more famous than the first one in Chumash:
Rebbi Yitzchak said: The Torah should have commenced with the verse (Shemos 12:2) “This month shall be unto you the first of the months” which is the first commandment given to Israel. What is the reason, then, that it commences with the account of Creation? Because of the thought expressed in the text (Tehillim 111:6) “He declared to His people the strength of His works (i.e. He gave an account of the work of Creation), in order that He might give them the heritage of the nations.” For should the peoples of the world say to Israel, “You are robbers, because you took by force the lands of the seven nations of Canaan”, Israel may reply to them, “All the earth belongs to the Holy One, blessed be He; He created it and gave it to whom He pleased. When He willed, He gave it to them, and when He willed, He took it from them and gave it to us” (Bereishis 1:1).
There are many difficulties in understanding this Rashi. While it is surely important to affirm that Eretz Yisrael belongs to the Jews, it really is striking that this should be the very beginning of the Chumash. Secondly, while we believe this to be the case, we have a hard time convincing the nations of the world that this is so; it is very hard to take the Chumash with the Rashi, bring it to the nations, and point a finger and say, “here, you see it says it belongs to us!” So, what's the point of it? They will never be convinced by a verse of Chumash and Rashi, while we don't need to be convinced. So, who is it written for?
We also need to understand Rashi in a way that this is not telling us something that we would consider as ‘cutting a corner’; i.e. that Hashem is doing something that is ‘technically correct’ but not really appropriate. After all, don’t we all feel that things that are rightfully people's should be theirs, even if there is legal recourse to expropriate it? So, what is being stated?
Let us contemplate Rashi’s question, when he asks that the Torah should start with ‘HaChodesh hazeh lachem’. This refers to the fact that the world really consists of two strata. There is a natural world where there is a process as to how change takes place; it's true physically, economically, and politically. That natural world is embedded in Bereishis. It means Hashem created a world that has a natural mechanism and, as such, is “tov maod - very good”, as-is. The world functioning in its natural process and organization is something Hashem wants.
But there is a second strata on top of that. Hashem instructed the Jewish people with a set of laws, obligations, and prohibitions that they must keep. This is a whole new set of behaviors coming from a higher order; they do not emanate from the realities of our world. The basic Noachide laws, on the other hand, emanate from and apply to the world around us. This is why they ‘make sense’ to us.
Torah, therefore, being the document of divine commandments, emanating from beyond our tangible universe, should start with the halachos, as expressed in ‘HaChodesh hazeh lachem’. The natural world and its inherent morality doesn’t need a ‘Torah’ to express it. This is the essence of Rashi’s question.
Rashi’s answer is that even in the so-called natural world, there is one point that does not fit into the natural process; the natural laws of economics, society, politics, do not govern it, as such. When Chazal tell us that every country in the world has an angel that is in charge of it except for Eretz Yisroel (see Rabbeinu Bachya, Devarim 31:15), it refers to this point. It means that in all the countries in the world the natural order dominates, while the Divine hand is in the background. Eretz Yisroel is the exception. Therefore, in the very act of creation wherein Hashem set up the natural order which includes nations having homelands and geographical locations that they exist in, an exception was built in. Eretz Yisroel is just like the Beis Hamikdash which was able to accommodate many more people than its physical area could accommodate (Avos 5:5); this area was ‘extraterritorial’, i.e. it belonged to a different order and was not part of physical nature.
This means that Israel’s lands are not allocated merely by historical record, but rather by Divine will, as generated by righteous conduct. A rather famous example of this was when the Kushites came to live in Israel and continued their quasi-idol worship that they had gotten used to. Punishment came swiftly, and they desperately sent to the Jews asking them what's appropriate to worship and not to worship in Israel (Melachim 2, 17:25). Eretz Yisrael is a country that is much more in tune with the spiritual world, and the Jewish nation's performance is what dictates its hold on it.
Thus, the first passuk is telling us that Eretz Yisrael belongs to no one; it belongs to Hashem. It is not, and never was, part of the natural order. Those who fulfill His will have it, and if they cease to do so, they lose it. This is true about the nations of the world - when they sinned excessively, they lost Eretz Yisrael. But it also means that we, Klal Yisroel, maintain possession of it or lose it based on merit.
In effect, this passage is telling us that Hashem created a world to run naturally, but the creator leaves His mark in one place. Like an artist who paints a beautiful painting of some scene or other, and signs his name in the corner, so too Hashem has ‘painted’ a picture of a natural world, but in a corner his name is ’signed’. That corner is Eretz Yisroel. Nations lived there and were evicted from there based on their performance. This passage is not here to convince the nations of the world that we're right. It is there to teach us that the reason why we have Eretz Yisroel is because someone else did not deserve it, and we have it only because we deserved it.
This places the great burden on us to do what it takes to keep this land which is, in essence, Hashem’s land.