What is 'the matter' to which the Pasuk is referring?
Ramban: It is referring to the promise that Hashem made to Avraham at the B'ris bein ha'Besarim 1 - "Ki Ger Yih'yeh Zar'acha be'Eretz Lo lahem ... ve'Dor Revi'i Yashuvu Heinah" (' ... that your offspring will be strangers in a land that is not theirs ... and the fourth generation will return here').
In Lech L'cha Bereishis, 15:16.
What is the gist of this Pasuk, which seems to be merely a repetition of the pevious one?
Seforno: What the Torah is now saying is that a. it is not on the merit of Yisrael that Hashem miraculously delivered the Cana'anim into our hands, rather, due to their sins, they deserved to suffer defeat at our hands 1 and not at His, and b. that we should earn the right to inherit the land by carrying out the fulfillment of Hashem's promise to Avraham Avinu. 2
Seforno: As David ha'Melech said in Shmuel 2, 24:14 "Niplah Na be'Yad Hashem, u've'Yad Adam Al Epolah!"
Seforno: In similar fashion to what Hashem said to Yeihu ben Nimshi in Melachim 2, 10:30, after he destroyed the house of Ach'av.
