The Lamanites appear to have come down from the land of Nephi at the end of the growing season, when the crops were still in the field in the land of Zarahemla. Thus "many of their fields were destroyed, for they were trodden down by the hosts of men" (Alma 3:2). John Sorenson writes:
In civilizations at such a level of technological development, armies were formed of nonprofessional militia. . . .The demand for manpower to carry on agriculture provided the most stringent limit on maintaining armies . . . When an army did have to be kept in battle readiness, an added burden fell on the men who were still cultivating. . . . But unavoidably, most of those serving in an army had to meet farming's demands during part of the year. . . .
Anywhere in the tropics, rain characterizes approximately half the year--the same season when the crops are growing--with resulting muddy trails and swollen streams to cross. In all likelihood, the only time when Alma and his forces could have waded across the river Sidon, fighting as they went (see Alma 2:27), would have been in the drier part of the year. Furthermore, had armies been fighting during the rains, they would have suffered significantly while traveling, camping, or fighting, for that time can be uncomfortably cool and unhealthy. Typically the Lamanites traveled virtually naked to reach the Nephites (see Enos 1:20; Alma 3:5; 43:20, 37). . . .
Of course, there could be exceptions. Regions varied in climate; certain places and times would have permitted at least limited fighting other than at the normal dry time, although we must assume that planned major campaigns had to follow the general rule. (John L Sorenson, "Seasonality of Warfare in the Book of Mormon and in Mesoamerica," in Warfare in the Book of Mormon, FARMS, 1990, pp. 447-448).
Thus one might ask, What is happening here if the battles are occurring during the growing (rainy) season? Apparently this was not considered to be a major campaign from primarily an outside force (the Lamanites). Those who supported the Amlicite desire for kingship were "an enemy within." They had become "powerful" and apparently thought that this revolt or "coup" attempt would amount to a very quick change in government. Although the Lamanites did help, they apparently were a little late, thus nullifying a fuller initial effect than the combined Amlicite-Lamanite forces might have had upon the hill Amnihu. This delay could have been caused by the rainy season and apparently saved the Nephites, for it allowed room for the strategy of Alma and the power of the Lord to influence the outcome. The Amlicites and Lamanites were thus routed rather than being heirs to spoil. Rather than achieving power, the people of Zarahemla who gave encouragement to the Amlicite movement brought all the Nephite people down into a dire state of weakness with a meager harvest. [Alan C. Miner, Personal Notes]