Coriantumr does the unexpected, he makes a rapid strike into Zarahemla. We are not told the path of his approach, but the most direct would have taken him past the typical defensive positions of the Manti line. The speed of his march is probably the most important part of this bold move, and in order to have an army move quickly, Coriantumr would need to take the most direct route. This suggests that he simply passed by Manti and the other defensive cities and drove straight for Zarahemla.
This was a very bold move, because it put a Nephite army at his back. Had Zarahemla been unattainable, Coriantumr’s force would be in very dire straits, with no defensive city available to them, and the Nephite army bearing down from the rear. As it was, the weakened state of Zarahemla made it vulnerable, and Coriantumr’s possession of it gave him resources and a strong defensive position should the Nephites come against him. In addition, the loss of the central city and the chief judge would have severely disrupted the government and resulted in some disorder in the Nephite ranks, and perhaps delayed the arrival of any assistance. Coriantumr has employed the blitzkrieg tactic to good advantage.
Verse 19 reminds us that Zarahemla had no time to assemble its armies. The danger of Lamanite attack required some standing military in the land of Zarahemla, but most of that army was on the periphery. The bulk of Mesoamerican armies, and certainly that of Zarahemla, was made up of the men of the hegemony who responded to the call to arms. Since Coriantum’s attack was so rapid, there was no time to assemble the men in from the various fields and outlying cities and towns, and therefore Coriantumr not only faced a reduced number of defenders, but the majority of the well-trained Nephite force was far from his location.