Daniel Peterson notes that Moronihah, the supreme commander of the Nephite armies, evidently faced with a shortage of troops, had chosen to place those that were available to him on the perimeter of Nephite territory. That is, they were stationed all about the lands of the Nephites, on the borders. Under the circumstances, this was the reasonable thing to do. But it also had its risks, since, as in the case of the notorious Maginot line in World war II, any invader who could get past the initial resistance at the border would find few obstacles from that point on.
And that is precisely what happened. Coriantumr launched a kind of blitzkrieg, piercing the Nephite border defenses, and seized the capital city of Zarahemla. The conquest was quick and, for the moment, total. Pacumeni, the chief judge, was killed. Perhaps somewhat astonished himself at the scope of his victory, Coriantumr then marched toward the other chief Nephite city, Bountiful. En route, he mopped up what disorganized opposition he encountered--the Nephites having been taken so completely by surprise that they did not even have time to mobilize a challenge to him.
But Coriantumr would not reach Bountiful. As Hugh Nibley points out, his drive "had been successful because it was completely unexpected; and it had been unexpected because it was utterly foolish." (Hugh W. Nibley, Since Cumorah, p. 367). It was utterly foolish because, having penetrated the Nephite defense on the circumference and seized the center of the land, Coriantumr was now, by the very nature of his own success, surrounded:
The Lamanites could not retreat either way, neither on the north, nor on the south, nor on the east, nor on the west, for they were surrounded on every hand by the Nephites. And thus had Coriantumr plunged the Lamanites into the midst of the Nephites, insomuch that they were in the power of the Nephites, and he himself was slain, and the Lamanites did yield themselves into the hands of the Nephites. (Helaman 1:31-32)
[Daniel C. Peterson, "Their Own Worst Enemies," in Studies in Scripture: Book of Mormon, Part 2, pp. 95-96]
Geographical [Theory Map]: Helaman 1:1-27 Coriantumr Takes the City of Zarahemla--Marches towards Bountiful (41st Year)
Helaman 1:28-29 [Lehi Heads] Coriantumr (41st Year)
Helaman 1:30-33 [Moronihah & Lehi] Defeat Coriantumr (41st Year)