Cumeni is the next logical military target. It is the next city in the line held by the Lamanites. The plan for the attack on Cumeni is for a complete siege. This means that the city had to be cut off from any supplies. The Nephite spies had been able to determine that a supply line was approaching Cumeni, and they surprise the inhabitants by cutting off that city with a night maneuver.
While this night move is also very difficult, it is made easier in that they force does not have to remain in battle formations. They are able to take their place and wait. They are able to make this move because they have received so many reinforcements from Zarahemla. It is doubtful that prior to the reinforcements that Helaman had sufficient men to surround Cumeni.
The choice of timing shows that Helaman understood the problems of holding a city. While defensible, the cities nevertheless required food to function, and as a front-line city after the fall of Antiparah, they could not safely till the fields around Cumeni. The city had to be supplied from the other Lamanite-held cities. The arrival of a new supply train indicated to Helaman that it had been some time since the last, and that food supplies were lower than they had been. By cutting off this supply line just as it was to arrive, they cut off Cumeni when it has fewer provisions for a long siege. Once again we see the Book of Mormon presaging military techniques that were documented for the later Aztecs (Hassig 1988, p. 107).