(Isa. 11:12–13)
It is important to know biblical history in order to understand how significant it is that Judah and Ephraim will be at peace in that day (Isa. 11:12–13). In biblical times, Ephraim, the Northern Kingdom (also called Israel), and Judah, the Southern Kingdom, often fought each other (Isa. 11:13), as we have seen (Isa. 7–8). Things will be otherwise in the Millennium, when they will come together amicably. As the scattered bones of a body will be reunited into one body, so scattered Israelites will become one nation under one king (see Ezek. 37:1–14).
(Keith A Meservy, Studies in Scripture, Vol. 4, ed. Kent P. Jackson [Salt Lake City: Deseret Book Co., 1993], 103.)
The entire verse recalls the tensions and hostilities that existed between Ephraim (the Northern Kingdom) and Judah (the Southern Kingdom), beginning with Jeroboam’s break with Rehoboam (1 Kgs. 11–12). God’s latter-day gathering will result in the uniting of the kingdoms of Judah and Israel, made possible because the gospel teaches peace and love and because the Messiah, who is the true king of Israel and Judah, rules with justice and righteousness (11:1–5). The Book of Mormon also plays a prominent role in reuniting Ephraim and Judah (Ezek. 37:15–28).
(Donald W. Parry, Jay A. Parry, and Tina M. Peterson, Understanding Isaiah [Salt Lake City: Deseret Book Co., 1998], 124.)
[Isaiah 11:13–14.] These verses describe a return to a united kingdom of Israel as it was before the division following the reign of Solomon. Ephraim was the name by which the northern 10? tribes were called, and Judah consisted of the southern 1? tribes. Ephraim will at the time of this reuniting occupy the promised land of Joseph, the Americas (see 3 Nephi 15:12–13). This 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; and 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 north of Edom on the east side of the Jordan. 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 departed from between Ephraim and Judah to bring about a united rule.
(Monte S. Nyman, Great Are the Words of Isaiah [Salt Lake City: Bookcraft, 1980], 75–76.)
Who is Judah? The Jews. Who is Ephraim? Those of the birthright, the leaders of the kingdom, the Church. This harmony between Judah and Ephraim is something never before experienced, due to the early envy and jealousy inherited from Judah himself toward the chosen son of Joseph. News releases of the day confirm and fulfill this latter-day prophecy. Judah has given Ephraim a position on its famous Mount Olivet for a historical monument of peace, the Orson Hyde Memorial. The Jerusalem Post referred to President Spencer W. Kimball as a “prophet in the line of Abraham and Moses.” (Church News, Dec. 29, 1979, 5.)
(L. La Mar Adams, The Living Message of Isaiah [Salt Lake City: Deseret Book Co., 1981], 73–74.)