Tafsiran Ulangan 33:5: Rashi, Chizkuni, Da'at Zekenim & Ramban

ויהי AND HE WAS — i.e. the Holy One, blessed be He, was

ויהי, “he became,” i.e. Moses; בישורון מלך בהתאסף ראשי עם, “King in Yeshurun, when the leaders of the people assembled.” This was recorded in Deuteronomy 4,10: באמור ה' אלי הקהל את העם ואשמיעם את דברי, when the Lord said to me: “assemble the people so that I can let them hear My words.”It is up to us to keep the Torah which He commanded us, and after having accepted it, it became a source of blessing for us.

ויהי בישורון מלך בהתאסף ראשי עם, “and there was a king in Jeshurun when the heads of the people were gathered.” When the Jewish people are truly united and relate to one another as true brothers, then G–d is truly King over them. When they quarrel among themselves then G–d cannot truly be seen as being their King.