Or HaChaim on Deuteronomy 31:17/אור החיים על דברים ל״א:י״ז

1 ועזבתים והסתרתי פני מהם, "and I will abandon them and hide My face from them, etc." The meaning is that if G'd were to allow Himself to dwell on the troubles which will befall His people, His mercy would be stirred and He would banish His anger. This is why He informs the people in advance that if they were to turn to idols, He, G'd, would turn His face away from them so as not to be tempted to show them compassion.

2ואמר ביום ההוא, "and the people of Israel will say on that day, etc." They will be intelligent enough to conclude that all the troubles which suddenly overtook them must be due to G'd having deliberately left their midst. Had G'd not abandoned them it would be impossible that He would tolerate such treatment of His people. Compare what the prophet said in Judges 10,11. We gain the impression from there that G'd Himself had become fed up with the sufferings His people had to endure.