The Holy One Blessed Be He, Gave Us The Land
So that if the nations of the world come and say that the land belongs to them, asking how we took it from the Hittites, Amorites, Canaanites, Perizzites, Jebusites, and Girgashites—taking their land from them— he answers: No, everything belongs to the Holy One, blessed be He. The Holy One, blessed be He, created heaven and earth, created the entire world. At His will He gave it to them, and at His will He took it from them and gave it to us.

The Book of Genesis is called the Book of the Upright (Sefer HaYashar). It is the book of the patriarchs of the world, who conducted their entire lives according to uprightness and truth. For them, proper conduct (derech eretz) preceded the Torah; it is also called the Book of Proper Conduct. When a person sees how the patriarchs behaved, what happened with them, and how—he learns proper conduct, knowing how to educate himself and how to educate his children.
What a great privilege it is that the Holy One, blessed be He, began [the Torah] for us with Genesis. As Rashi says, the Torah exists for the sake of the 613 commandments. The very first commandment we were commanded was the sanctification of the new month: "This month shall be unto you the beginning of months; it shall be the first month of the year to you". This is how the Holy One, blessed be He, taught Moshe our Teacher. He showed him the new moon and instructed him on how to sanctify the month—that was the first commandment.
Why then did He begin with "In the beginning the L-rd created the heaven and the earth"? Rashi explains: "The strength of His works He related to His people". So that if the nations of the world come and say that the land belongs to them, asking how we took it from the Hittites, Amorites, Canaanites, Perizzites, Jebusites, and Girgashites—taking their land from them— he answers: No, everything belongs to the Holy One, blessed be He. The Holy One, blessed be He, created heaven and earth, created the entire world. At His will He gave it to them, and at His will He took it from them and gave it to us. They did not conduct themselves properly, so He took it—you see, it was not us; the Holy One, blessed be He, gave us the land. This is "The strength of His works He related to His people, to give them the inheritance of the nations.".