Alma 24:5 says that Ammon and his brethren "came forth to the land of Midian," but where did they come forth from? Aaron was originally in the land of Nephi (Alma 23:16). Ammon, on the other hand was originally in the land of Ishmael (Alma 21:8; 22:1). However, according to Alma 23:4-5, it says that after the king had sent forth his proclamation, "Aaron and his brethren went forth from city to city." Thus it seems that although Aaron, Ammon, and their brethren were going from city to city doing missionary work, Aaron's base was probably in the local land of Nephi and Ammon's base was in the land of Ishmael.
The missionaries gathered for a strategy session in the face of preparations for war by the unconverted people against the converted ones. They first gathered to "the land of Midian," a place only mentioned in Alma 24:5 and nowhere else. From there they moved to the land of Ishmael. We might presume that the land of Midian was a convenient gathering point from the areas where the different missionaries had been serving, more especially between the land of Ishmael and the local land of Nephi. [Alan C. Miner, Personal Notes] [See Geographical Theory Maps]
Geographical Theory Map: Alma 24:5 Ammon Meets with His Brethren in Midian and Ishmael (10th Year)