In the opening words of his commentary, Rashi, quoting his father R' Yitzchak, notes that the Torah does not relate the first mitzvah given to the Jewish People until the 12th chapter of Shemos. Before reaching that point, we experience history from the creation of the world through the dawn of our redemption from Egypt. The Torah presents this material so that Hashem can convey a message supporting our right to Eretz Yisroel.
Whenever the other nations claim that the Jews stole their land, we can simply point to this history in the Torah and proclaim that Hashem as Creator can choose to give it to whomever He wants. As we witness daily, they always accept this truth and go quietly away.

This unfortunately has never been their reaction and, thus, cannot be what Rashi means. The "historical" message of the Torah must therefore be directed at someone else.
The actual counter to their claim is contained in the verse Rashi quotes from Tehilim (116:6), "The power of His acts He told to His people in order to give them the estate of nations." The emphasis of this verse is on what Hashem has told us. If we listen, learn and truly believe that Eretz Yisroel belongs to us, then nothing else and no one else really matters. By living here, studying here, and fulfilling the mitzvot linked to the land, we show that we, His people, accept this answer. Our right to Eretz Yisroel starts with our belief in our own entitlement, which in turn rests on the strength of our belief in Hashem and His Torah.