Mormon explains that
...it was only the distance of a day and a half’s journey...on the line Bountiful and the land Desolation, from the east to the west sea; and thus the land of Nephi and the land of Zarahemla were nearly surrounded by water, there being a small neck of land between the land northward and the land southward.
Mormon records that the distance to cross the strip of land was a day and a half of walking or by an animal--he did not specify which. Patrolling that length of land would take a small army depending on how the Nephites did it! They did do it, though.
Mormon wanted to hold the Lamanites at that place. With so little ground to cover it was easier to protect and defend that confined space. It is evident that naval warfare was not something the Nephites and Lamanites contrived though surrounded by water. They utilized hand-to-hand combat--maybe slings were used.
Armed with such tools and the confidence to boot, the Nephites prepared under Mormon's lead to defend against the Lamanite invasion. In 361 and 362, A. D. the Lamanites warred with the Nephites and did not conquer them. Each time the Lamanites attacked, the Nephites repelled them! Such success among the Nephites did not lead to gratitude as Mormon recorded, but to pride and self-righteous behavior.
Mormon tolerated many things from his men, but when they forswore themselves of all that that he had vowed not to, Mormon had his fill of wickedness. He complained
...they began to boast in their own strength, and began to swear before the heavens that they would avenge themselves of the blood of their brethren who had been slain by their enemies. And they did swear by the heavens, and also by the throne of God, that they would go up to battle against their enemies, and would cut them off from the face of the land. And it came to pass that I, Mormon, did utterly refuse from this time forth to be a commander and a leader of this people, because of their wickedness and abomination."Mormon 3:9-11
Swearing by heaven and the throne of God pushed Mormon to his career's end as their general because of the depth to which he held his covenant with Jesus Christ to not dishonor those things.
Nephi described the gospel of Jesus Christ as a strait gate, a narrow gate. The only way to get in is through this gate by following the commands of God precisely. In 2 Nephi 31:18-21 , Nephi states that
"this strait and narrow path which leads to eternal life; yea, ye have entered in by the gate; ye have done according to the commandments of the Father and the Son; and ye have received the Holy Ghost, which witnesses of the Father and the Son, unto the fulfilling of the promise which he hath made, that if ye entered in by the way ye should receive...after ye have gotten into this strait and narrow path, ...ye have not come thus far save it were by the word of Christ with unshaken faith in him, relying wholly upon the merits of him who is mighty to save.
...[Y]e must press forward with a steadfastness in Christ, having a perfect brightness of hope, and a love of God and of all men. Wherefore, if ye shall press forward, feasting upon the word of Christ, and endure to the end, behold, thus saith the Father: Ye shall have eternal life... [T]here is none other way nor name given under heaven whereby man can be saved in the kingdom of God...."
The narrow path that Mormon walked could not include the swearing upon sacred things. He retired his general’s regalia at the Narrow Neck of Land. It was a symbolic statement. The defeat of the Lamanites occurred at the Narrow Neck and the way became too narrow for the soldiers under Mormon's command--or should I say the path became too broad for Mormon to allow further insult to injury.
As general, Mormon decided to draw a line and not accept every type of behavior thrown at him. If he could not garner the respect of his men enough to cease defiling deity, he would leave them to their own designs--he would try to anyway.
The Significance of Desolation in the Book of Mormon Heritage
Notice the importance of Desolation to the Nephites as Mormon recorded it in the Book of Mormon. Desolation has received many mentions in the book. At first glance, the reader might disregard the significance of the word at all.
Desolation is the name of a land as stated before in conjunction with the city which headed the passage into the Northern lands then occupied by the Nephite nation. The land of Desolation received its appellation from the Mulekites, another group of people besides the ancestors of the Nephites who journeyed to the ancient Americas before the captivity of Judah by Babylon.
This group of people thus named the land because of the number of human skeletal remains that they found upon the land and the lack of trees in the land upon arriving there following their journey.
It follows that the land south of Desolation was called Bountiful because of its bounty of animals and game among other things. Desolation existed as its name denotes--because of a great ecological purging and destruction in the land Northward.
Mormon highlights the noteworthy voyages of:
Hagoth, he being an exceedingly curious man, therefore he went forth and built him an exceedingly large ship, on the borders of the land Bountiful, by the land Desolation, and launched it forth into the west sea, by the narrow neck which led into the land northward. Alma 63:5
Desolation is the land Northward but throughout the record, Mormon refers to it depending on circumstance. In fact, it seems that Mormon does much to keep from naming the incidental places by calling places the "land of Zarahemla" rather than specify a suburb of the city proper.
He does this when referring to the birthplace of his coming Lord Jesus by writing at Jerusalem rather than mentioning the suburb Bethlehem when copying out the speech Alma gave at a city called Gideon. God would have revealed to Nephi that city when considering all the other things he revealed to the man!
Also, Lachoneous, the chief judge during the seven-year-siege (though it did not last that long), used the land of Desolation as a line of demarcation on the north and Zarahemla on the south to wait out the Gadianton robbers who afflicted both Nephites and Lamanites the years 19 to 22 A. D.
Having to use the border of the Land of Desolation as the holding point again takes on significance for the Nephites. This time, however, the Nephites would be on the northern side of the border instead of the southern.
This time, the people would be in the part that was actually Desolation. This time, the land would describe their souls as well as their location because they refused to heed the counsel of God through Mormon.