1)
What is the meaning of "Yadber Amim Tachteinu"?
1.
Rashi: He will put Dever (plague) on the nations in place of our souls, so His anger will be cooled on them, and we will be saved - "Nasati Chafrecha Mitzrayim" (Yeshayah 43:3).
2.
Rashi citing Menachem: "Yadber" it is an expression of conducting. Radak - He will lead them from the end of the land until Yerushalayim, to put them under us.
3.
Malbim: He will put the nations under us regarding conducting Mishpat - "v'Shafat Bein ha'Goyim" (Yeshayah 2:4).
2)
Why does it say "u'Le'umim Tachas Ragleinu"?
1.
Malbim: Regarding creed and Torah [they will be under us] - "Ki mi'Tziyon Tetzei Sorah" (Yeshayah 2:3)
