In Alma 50:11, the phrase "from the west sea running by the head of the river Sidon" seems to describe the defensive line set up. Here we should expect some kind of physical setting that was defendable: mountains, rivers, passes, narrow borders by the sea, etc.
“The Nephites Possessing All the Land Northward”
John Sorenson writes:
The Sierra Madre mountains form an all but impossible barrier to regular travel between the Pacific seacoast and the interior depression of Chiapas [Sorenson's proposed greater land of Zarahemla] all along it's southern extremity, with one noteworthy exception: a pass links the upper tributaries region of the Grijalva River [Sorenson's river Sidon] via the town of Motozintla to the wide, rich foothill and [Pacific] coastland strip known as the Soconusco. In the opposite direction from Motozintla, a narrow river valley leads the other way down toward the Grijalva, [and the interior depression of Chiapas] . . . and toward the Chicomuselo area, the proposed site for the city of Judea. . . . [Thus the location of the city of] Antiparah fits well near the site of Motozintla. . . .
We see why [Antiparah would fit this scenario] by examining the Nephite recapture of Antiparah. Antipus and Helaman, the Nephite leaders on this front, used "a stratagem" to get the Lamanites to come out from within the city's defenses. They sent a small party past the place, teasing the Lamanites to pursue them. The group's destination was meant to be obvious by the route it took: "as if we were going to the city beyond, in the borders by the seashore" (Alma 56:30-31). So Antiparah lay in or near a pass on a route that led down toward the shore from Antiparah on the one hand and toward Zarahemla via Judea on the other. A band of men moving seaward within sight of a defense location in the river valley near Motozintla would obviously be headed over the nearby pass and down [to the Pacific coast] to Izapa or some other city in the Soconusco region. [John L. Sorenson, An Ancient American Setting for the Book of Mormon, pp. 257-258, emphasis added]
Izapa might qualify (both internally and externally) as "the city beyond, in the borders by the seashore" (Alma 56:31) because, at least in Sorenson's model, Izapa could well have been connected in some manner to Moroni's military strategy of about 72 B.C. in which he "cut off all the strongholds of the Lamanites . . . on the west, fortifying the line between the Nephites and the Lamanites, between the land of Zarahemla and the land of Nephi, from the west sea, running by the head of the river Sidon (Alma 50:11, emphasis added) The phrase, "from the west sea, running by the head of the river Sidon," correlates very well with Sorenson's statement about the possible real world military importance of Izapa's location. Sorenson says above: "a pass links the upper tributaries region of the Grijalva River [his proposed river Sidon] via the town of Motozintla to the wide, rich foothill and [Pacific] coastland strip known as the Soconusco [of which Izapa was a principle city]." Thus, according to Sorenson's Book of Mormon geographical theory, to defend Izapa was to defend not only the coastal travel route towards the Isthmus of Tehuantepec (the Narrow Neck of Land) and Veracruz (the Land Northward), but to defend coastal access to the headwaters of the Grijalva (Sidon) river, which in turn led down into the Chiapas depression (the Land of Zarahemla). This made the region of Izapa a key fortification point in the Nephite military strategy to prevent the Lamanites from "marching into the land northward" (Alma 52:2) that the Lamanites "should have power to harass [the Nephites in Zarahemla] on every side" (Alma 52:9). [Alan C. Miner, Personal Notes]
Geographical [Theory Map]: Alma 50:11 Moroni's Fortification Line (20th Year)
Geographical [Theory Map]: Alma 50:11 Moroni's Fortification Line (20th Year)
Geographical [Theory Map]: Alma 50:11 Moroni's Fortification Line (20th Year)--on the West
Alma 50:11 ([Illustration]) Nephites possess all the Land Northward of the Land Bountiful 20th year