Alma 43 wrote:27 And it came to pass that Moroni caused that his army should be secreted in the valley which was near the bank of the river Sidon, which was on the west of the river Sidon in the wilderness.
31 Therefore, he divided his army and brought a part over into the valley, and concealed them on the east, and on the south of the hill Riplah;
32 And the remainder he concealed in the west valley, on the west of the river Sidon, and so down into the borders of the land Manti.
33 And thus having placed his army according to his desire, he was prepared to meet them.
Captain Moroni positioned his army east of river Sidon (runs north & south) in secret tactical locations in preparation to ambush the Lamanites who would come from the northeast quarter of Manti. The Nephite army was essentially “divided” into two units in which one would be stationed on the east of the river and the other on the west being secretly positioned in a valley close to the bank of the river.
The army on the east side of the river positioned themselves at hill Riplah being concealed on the south and east portions of the hill. So, the Nephites were positioned on both sides of the river but on the east they were concealed behind a hill awaiting the Lamanites to come from the north and then spring the trap as they attempt to cross the river. It was a brilliant set-up in which captain Moroni could move in and encircle the Lamanites by entrapping them on both sides of the river!
Alma 43 wrote:34 And it came to pass that the Lamanites came up on the north of the hill, where a part of the army of Moroni was concealed.
35 And as the Lamanites had passed the hill Riplah, and came into the valley, and began to cross the river Sidon, the army which was concealed on the south of the hill, which was led by a man whose name was Lehi, and he led his army forth and encircled the Lamanites about on the east in their rear.
36 And it came to pass that the Lamanites, when they saw the Nephites coming upon them in their rear, turned them about and began to contend with the army of Lehi.
So, as we see, the Lamanites did come from the north and passed the hill Riplah in order to cross the river at that point and enter the land of Zarahemla in hopes of conquering the Nephites. But they were met with disappointment because Moroni sprung his trap! Lehi and his men came out from hiding and before the Lamanites realized it they were surrounded because hill Amnihu was suddenly transformed into a mighty Nephite army and their only recourse was to turn and fight or flee to the river!
Alma 43 wrote:39 And it came to pass that the Lamanites became frightened, because of the great destruction among them, even until they began to flee towards the river Sidon.
40 And they were pursued by Lehi and his men; and they were driven by Lehi into the waters of Sidon, and they crossed the waters of Sidon. And Lehi retained his armies upon the bank of the river Sidon that they should not cross.
41 And it came to pass that Moroni and his army met the Lamanites in the valley, on the other side of the river Sidon, and began to fall upon them and to slay them.
42 And the Lamanites did flee again before them, towards the land of Manti; and they were met again by the armies of Moroni.
51 Now, the Lamanites were more numerous, yea, by more than double the number of the Nephites; nevertheless, they were driven insomuch that they were gathered together in one body in the valley, upon the bank by the river Sidon.
52 Therefore the armies of Moroni encircled them about, yea, even on both sides of the river, for behold, on the east were the men of Lehi.
53 Therefore when Zerahemnah saw the men of Lehi on the east of the river Sidon, and the armies of Moroni on the west of the river Sidon, that they were encircled about by the Nephites, they were struck with terror.
Thus we see the Lamanites had their backs on both sides of the river having to face the Nephites with Moroni on one side and Lehi on the other -- the Lamanites were effectively hemmed in and were no match against the armored Nephite soldiers. The only recourse the Lamanites had was to flee northward to Manti but they were thwarted in doing so by Moroni on the west side of the river. It was checkmate! A brilliant move on the part of captain Moroni
Joseph Smith’s storytelling while his face was buried in a hat.
Interestingly enough, we are informed in chapter 44 that the number of dead was so great that there was no body count for either side but the dead were conveniently cast into the river. We may safely assume that the brilliant author of this war story imagined those bodies quickly floating downstream on the Sidon current where as the text says, “they have gone forth and are buried in the depths of the sea.”