Rashi cites the Midrash which explains that Moshe was apprehensive that the merit which Og earned in assisting Avraham Avinu would shield him from defeat. How did Og assist Avraham? When Lot, Avraham’s nephew, was captured, Og brought the news to Avraham. The Midrash explains that the merit did not help Og, because his intention was actually to hurt Avraham, not to help him. He hoped that by informing Avraham of Lot’s imprisonment, he would entice Avraham to be drawn into a war which would eventually cost Avraham his life. Thus Og would be enabled to marry Sarah. Since Og’s…
