According to Alma 27:23, the Nephites make a promise to the Anti-Nephi-Lehies that "we will set our armies between the land Jershon and the land Nephi, that we may protect our brethren in the land Jerhson." This statement implies a relatively short distance between the two lands. But just how far was it from the borders of southern Jershon to the land of Nephi? Notice should be taken of the fact that Mormon seems to dwell on these relationships in the chapters which follow. In the subsequent chapters we will see that at least the land of Antionum was positioned in between the Nephite controlled area and the Lamanite controlled areas: "Now the Zoramites had gathered themselves together in a land which they called Antionum, which was east of the land of Zarahemla, which lay nearly bordering upon the seashore, which was south of the land of Jershon, which also bordered upon the wilderness south, which wilderness was full of the Lamanites." (Alma 31:3) . The reader should note that while Alma 31:3 mentions a southern border of the land of Antionum as being occupied by "Lamanites," Alma 27:23 specifically implies that the Nephite armies were located between the southern border of the land of Jershon and "the land Nephi." This not only implies that the Nephite armies occupied the land of Antionum (at the time of Alma 27:23), but that the general land of Nephi stretched to near the southern border of the land of Jershon. [Alan C. Miner, Personal Notes]
“We Will Guard Them from on Condition That They Will Give Us a Portion of Their Substance That We May Maintain Our Armies”
According to an article by John Welch, the only Book of Mormon group given an exemption from military service was the famous people of Ammon. In repenting of their previous shedding of blood, they had sworn an oath that they would never again take up arms (see Alma 24:11-13). After they arrived in the land of Zarahemla, they were granted an extraordinary exemption from active military duty if they would help to sustain the Nephite armies with provisions (see Alma 27:23-24). Surprisingly, the grant of this exceptional privilege was consistent with ancient Israelite law.
Normally one duty of ancient peoples was to take up arms in defense of their tribe or nation. Saul called "all Israel" to take up arms against the Ammonites and the Amalekites (see 1 Samuel 11:1-11; 15:4). Threats and curses were pronounced upon anyone who would not join in the battle. Once, Saul sent messengers to marshall the troops; he symbolically cut a yoke of oxen into pieces in view of the people and proclaimed, "Whosoever cometh not forth after Saul and after Samuel, so shall it be done unto his oxen" (1 Samuel 11:7).
Why, then, were the able-bodied Ammonites granted exemption? There may be several reasons.
1. The absolute duty to go to war applied only in fighting against an enemy. (Deuteronomy 20:1-2) This was interpreted in the Talmud as not applying in a conflict against other Israelites: "'Against your enemies' but not against your brethren, not Judah against Simeon nor Simeon against Benjamin." (The Talmud was a text later than the departure of Lehi from Jerusalem, but it often reflected older material. It was not translated into English until after the Book of Mormon was in print.) A similar feeling may be reflected in the Ammonite reluctance to "take up arms against their brethren" (Alma 24:6,18; 27:23)
2. The laws of Deuteronomy afforded humanitarian exemptions for those who were "fearful or fainthearted" (Deuteronomy 20:5-9; 24:5). Since everyone going into battle was likely "fearful and fainthearted," the Talmud explains, this "alludes to one who is afraid because of the transgressions he had committed." Certainly the Nephites would have recognized the profound fears of the Ammonites.
3. The men who remained at home, however, continued to support the war behind the lines. The Talmud holds that those who are exempted from actual fighting are exempted from those particular duties, but not from serving in the rear: 'They must furnish water and food and repair the roads.'"
Thus, the exemption granted to the Ammonites was logical, religiously motivated, and consistent with the spirit of ancient Israelite law. [John W. Welch, "Exemption From Military Duty," in Reexploring the Book of Mormon, F.A.R.M.S., pp. 189-191]