Although a year marker passes between the delivery of the letter of intent to come to war and the actual beginning of hostilities, we should not therefore presume that an entire year has passed between the delivery of the letter and the delivery of the army. The most probable timing is that the letter is sent in the end of the year, and the attack begins in the next. The timing of the attack is also likely to have been earlier in the year rather than later. The Nephites are in a defensive position, and one of the strategies for attacking such a position is to siege the location.
If this is part of the strategy, the idea of the attack would be to prevent the planting of the crops. Even if the current attack fails, which it did, they have weakened the position for the future by diminishing the available store of food. Certainly the defensive position had stocks of food, but the Lamanites are intent on a long-term battle, and would be looking to the future as well as the present. While warfare was typically fought after the harvest, the Lamanites would have a large land with greater supply capabilities than the concentrated Nephites. Therefore they could afford to have men out of the fields fighting, where that same condition would eventually weaken the Nephites.