Here we have four pieces of information. The first is that the Lamanites have moved their own armies. This is not simply a rebellion of Antionum, but the war will encompass part of the military might of the Lamanites. This process of combining forces with those of another city state was a common practice in Mesoamerican warfare. Indeed, part of the continued success of a large hegemony of city states was the ability to muster armies from a much larger land than that of the central city which was politically dominant. This became particularly true of the Aztec empire. The larger they became, the more difficult it was to overthrow them because of their ability to muster and provision large armies at distant locations.
The next information is that the Lamanites “came with their thousands.” It is quite possible that there really were multiple thousands of Lamanite soldiers, but we must also remember that numbers behave suspiciously in the Book of Mormon, particularly when used in a military context (see the comments on counts and estimates in the Book of Mormon following Alma 2:19).
These Lamanite troops entered Antionum. The rebellion of these form Nephites-now-become-Lamanites has led to a situation where they could call upon Lamanite armies. No doubt they will also raise troops of their own, but certainly they were under obligation to support the Lamanites. The presence of the Lamanites in Antionum is precisely the fear which led Alma to begin preaching to the Zoramites in the first place:
Alma (Alma 31:3-6)
3 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.
4 Now the Nephites greatly feared that the Zoramites would enter into a correspondence with the Lamanites, and that it would be the means of great loss on the part of the Nephites.
5 And now, as the preaching of the word had a great tendency to lead the people to do that which was just—yea, it had had more powerful effect upon the minds of the people than the sword, or anything else, which had happened unto them—therefore Alma thought it was expedient that they should try the virtue of the word of God.
6 Therefore he took Ammon, and Aaron, and Omner; and Himni he did leave in the church in Zarahemla; but the former three he took with him, and also Amulek and Zeezrom, who were at Melek; and he also took two of his sons.
Antionum held a very strategic position on the eastern flank of the land of Zarahemla, and opening Antionum as a support base for Lamanite military action opened a major breach in the geographic defenses of Zarahemla.
Lastly, we have the name of the military commander of the Lamanite forces, Zerahmenah. Up to this point in the Book of Mormon we have rarely seen the names of the commanders of the opposing forces, but we now see them with much greater frequency. We saw an earlier case when Amlici leads his armies against Alma, and eventually is defeated in hand to hand combat with Alma (Alma 2:29). The leader of an army in Mesoamerica was frequently either the king or a man who was close to the king. For instance, K’inich B’aaknal Chaak of Tonina “inherit[ed] the services of two aj k’uhuun who werved Ruler 2, but soon [were] joined by another called Aj Ch’anaah. This character would later use the yajaw k’ak’ “Lord of Fire’ title, probably denoting a key military position.” (Simon Martin and Nokolai Grube. Chronicle of the Maya Kings and Queens. Thames and Hudson, 2000. p. 181).
It is possible that the increasing detail of the campaigns and the presentation of the names of commanders is part of the same issue. Rulers become the de facto representative of not only their polity, but the ideology of the their polity. By presenting us with named people, Mormon can bring into higher focus the issues that are being addressed above and beyond the particular objective of each campaign.