These verses describe a return to a united kingdom of Israel as it was before the division following the reign of Solomon. The northern tribes were called Ephraim and Judah and half of Benjamin consisted of the southern kingdom. Ephraim will at the time of this reuniting occupy the promised land of Joseph—the Americas. “The Father hath given it unto [the remnant of Joseph]” (3 Nephi 15:12–13). Isaiah’s prophecy speaks of more than a geographical uniting; it is a spiritual and political reuniting. Abraham was promised the land “from the river of Egypt unto the great river, the river Euphrates” (Genesis 15:18), which has never as yet been completely occupied by Abraham’s children under one head. In Isaiah’s time the Philistines occupied the area of the present-day Gaza Strip on the Mediterranean coast, which falls within the geography of Abraham’s promised land. Edom, the descendants of Esau, occupied the area of Mt. Seir to the south and the east. Moab, the descendants of Lot and his eldest daughter, occupied the territory directly to the north of Edom on the east side of the Jordan River. Together, the Edomites and the Moabites represented the expansion of the land of Abraham on the east. The children of Ammon, the descendants of Lot and his younger daughter, occupied the land directly north of Moab on the east side of the Jordan. Today these all constitute the modern nation of Jordan.
That this prophecy is yet to be fulfilled is shown by the declaration that the children of Ammon will obey the house of Israel. None of this land east of the Jordan is under the rule of the house of Israel, nor has the envy yet fully departed from between Ephraim and Judah to bring about a united rule.