Alma 2 discusses Amlici's attempted insurrection. Alma leads the Nephite armies across the River Sidon to battle against Amlici and his army (the Amlicites). Although Alma's men prevail, the Amlicites who survive flee southward. Alma's spies follow them until they see the Amlicites join with a Lamanite army. Then the spies hurry back to camp, alert Alma, and a battle ensues.
You may be surprised to know that Alma 2 has generated a tremendous amount of discussion about Book of Mormon geography. The question is whether the Amlicites--and Alma's spies after them--crossed the River Sidon. Many people think they did, although the text doesn't say so. I disagree: I think the Amlicites and the spies stayed on the eastern side of Sidon.
The text frames the battle as part of a race to the city of Zarahemla: "except we make haste they obtain possession of our city, and our fathers, and our wives, and our children be slain." (Alma 2:25)
Verse 27 describes how the battle took place:
And behold, as they were crossing the river Sidon, the Lamanites and the Amlicites, being as numerous almost, as it were, as the sands of the sea, came upon them to destroy them.
I think this verse means Alma and his men were crossing the river from east to west, and the Lamanites came upon them from behind. Others think this verse means the Lamanites were already on the western bank. I've had numerous discussions on this point, and I think both interpretations are plausible, depending on what assumptions you make.
I adopted the first one because it makes more sense to me conceptually, and because the text specifies that Alma crossed the river initially to attack the Amlicites, and then crossed it again to defend Zarahemla, but it never mentions anyone else crossing the river.
In a way, this is geography minutia. You can read the debates online. For me,the point of this account is how much effort Alma exerted to protect and save the liberty of his people.
Still, here's how I envision the battle taking place: