RASHI - In the beginning: Said Rabbi Isaac: It was not necessary to begin the Torah except from “This month is to you,” (Exod. 12:2) which is the first commandment that the Israelites were commanded, (for the main purpose of the Torah is its commandments, and although several commandments are found in Genesis, e.g., circumcision and the prohibition of eating the thigh sinew, they could have been included together with the other commandments). Now for what reason did He commence with “In the beginning?” Because of [the verse] “The strength of His works He related to His people, to give them the inheritance of the nations” (Ps. 111:6). For if the nations of the world should say to Israel, “You are robbers, for you conquered by force the lands of the seven nations [of Canaan],” they will reply, "The entire earth belongs to the Holy One, blessed be He; He created it (this we learn from the story of the Creation) and gave it to whomever He deemed proper When He wished, He gave it to them, and when He wished, He took it away from them and gave it to us.