ציטירנדיקער פּירוש אויף תהילים 136:19

לְ֭סִיחוֹן מֶ֣לֶךְ הָאֱמֹרִ֑י “Sichon, the king of Amorites,” (Ps. 136:19) refers to the generation of Terach, father of Abraham. He was a chief minister of Nimrod as is written in the book of Divrei Hayamim. Just as Sichon was the doorway to the land of Canaan because everyone trusted his might, so too, everyone trusted the might of Terach. He was a great man and was referred to as the King of the Amorites, from the word Amirah, speaking. This was because he spoke libel against his son Abraham to Nimrod.

Therefore, our sages also said that it would have been just to recite songs of praise about Sihon and Og, as they did at the Exodus from Egypt regarding Pharaoh. However, King David came and recited songs of praise about them, as the verse says, “Sihon, king of the Amorites, His steadfast love is eternal; Og, king of Bashan, His steadfast love is eternal” [Psalms, 136:19–20].³⁰Yalkut Shimoni, Hukat, Remez, 765 (at the end).

this day This phrase can denote the future, if it is taken to mean, “ close to this day”. Actually, though, it refers to the conquest of the two Amorite kingdoms. Reuven, Gad, and half of Menasheh can certainly be called “Israel”, just as Scripture states elsewhere: “as Israel did to the land of his inheritance” [2: 12] After having said that “God is God; there is none beside Him” [: 35], Scripture proceeds to elaborate why He must be honored. The evidence of one’s own eyes forms the premise of the argument. Any clear-headed, unbiased person must realize from the evidence that God alone is God: in heaven above (i.e., above man) and upon the earth beneath (i.e., beneath man. Man is the standard of comparison, as in “The skies above you will be like brass…” [28: 23]). You, having seen all these wonders, having impressed it upon your hearts, must keep his commandments. Moreover, they are good for you, and for your children after you.