My reading of the Book of Mormon is that the armies were seasonal with a small core kept active during periods of warfare. The exception would be the final battles but many of those were fought in famine conditions and were sustained by plunder.
Since I doubt you will go to my website and actually read the page, I'll copy the pertinent part here:
Standing Armies and Provisions
Many portions of the Book of Mormon time-table deal with active warfare, and other sections address the fear of oncoming warfare, which requires certain preparations. These preparations can also provide background information that can be compared to actual ancient Mesoamerican history.
5 And now, Teancum saw that the Lamanites were determined to maintain those cities which they had taken, and those parts of the land which they had obtained possession of; and also seeing the enormity of their number, Teancum thought it was not expedient that he should attempt to attack them in their forts.
6 But he kept his men round about, as if making preparations for war; yea, and truly he was preparing to defend himself against them, by casting up walls round about and preparing places of resort.
7 And it came to pass that he kept thus preparing for war until Moroni had sent a large number of men to strengthen his army.
8 And Moroni also sent orders unto him that he should retain all the prisoners who fell into his hands; for as the Lamanites had taken many prisoners, that he should retain all the prisoners of the Lamanites as a ransom for those whom the Lamanites had taken.
9 And he also sent orders unto him that he should fortify the land Bountiful, and secure the narrow pass which led into the land northward, lest the Lamanites should obtain that point and should have power to harass them on every side.
10 And Moroni also sent unto him, desiring him that he would be faithful in maintaining that quarter of the land, and that he would seek every opportunity to scourge the Lamanites in that quarter, as much as was in his power, that perhaps he might take again by stratagem or some other way those cities which had been taken out of their hands; and that he also would fortify and strengthen the cities round about, which had not fallen into the hands of the Lamanites.
11 And he also said unto him, I would come unto you, but behold, the Lamanites are upon us in the borders of the land by the west sea; and behold, I go against them, therefore I cannot come unto you.
Alma 52, BC 66
2 And Moroni went to the city of Mulek with Lehi, and took command of the city and gave it unto Lehi. Now behold, this Lehi was a man who had been with Moroni in the more part of all his battles; and he was a man like unto Moroni, and they rejoiced in each other’s safety; yea, they were beloved by each other, and also beloved by all the people of Nephi.
3 And it came to pass that after the Lamanites had finished burying their dead and also the dead of the Nephites, they were marched back into the land Bountiful; and Teancum, by the orders of Moroni, caused that they should commence laboring in digging a ditch round about the land, or the city, Bountiful.
4 And he caused that they should build a breastwork of timbers upon the inner bank of the ditch; and they cast up dirt out of the ditch against the breastwork of timbers; and thus they did cause the Lamanites to labor until they had encircled the city of Bountiful round about with a strong wall of timbers and earth, to an exceeding height.
5 And this city became an exceeding stronghold ever after; and in this city they did guard the prisoners of the Lamanites; yea, even within a wall which they had caused them to build with their own hands. Now Moroni was compelled to cause the Lamanites to labor, because it was easy to guard them while at their labor; and he desired all his forces when he should make an attack upon the Lamanites.
6 And it came to pass that Moroni had thus gained a victory over one of the greatest of the armies of the Lamanites, and had obtained possession of the city of aMulek, which was one of the strongest holds of the Lamanites in the land of Nephi; and thus he had also built a stronghold to retain his prisoners.
7 And it came to pass that he did no more attempt a battle with the Lamanites in that year, but he did employ his men in preparing for war, yea, and in making fortifications to guard against the Lamanites, yea, and also delivering their women and their children from famine and affliction, and providing food for their armies.
8 And now it came to pass that the armies of the Lamanites, on the west sea, south, while in the absence of Moroni on account of some intrigue amongst the Nephites, which caused dissensions amongst them, had gained some ground over the Nephites, yea, insomuch that they had obtained possession of a number of their cities in that part of the land.
9 And thus because of iniquity amongst themselves, yea, because of dissensions and intrigue among themselves they were placed in the most dangerous circumstances.
Alma 53, BC 64
Both of these sections clearly describe a standing army, that is, an army that remains organized and structured while waiting for active battle. Again, this does not strike a modern reader as unreasonable, yet there is no evidence of such a practice in ancient Mesoamerica during the Book of Mormon time frame.
Payson Sheets explains in his essay “Warfare in Ancient Mesoamerica: A Summary View”, contained in the book Ancient Mesoamerican Warfare:
Warfare should not be seen as a monolithic repetitive entity among Mesoamerican societies. Rather, warfare varied considerably, depending on factors such as the degree of political centralization, presence or absence of competitive nearby polities, demographic and ecologic processes, ritualization of warfare, resource unpredictability, and economic and technological changes. Hassig deals with some of these factors, and I anticipate that future warfare research will both broaden and deepen them. Are there correlations between the degree of political centralizaton and the nature of warfare? In a general sense, there is a relationship, as both of the most highly urbanized and centralized civilizations, Teotihuacan and the Aztecs, waged war with standing armies against external polities. Hassig characterizes Teotihuacan and the Aztecs as meritocratic states with some social mobility for individuals based on their military successes. Both states were relatively innovative in military techniques and technology, and both developed large standing armies that expanded over much of Mesoamerica, but neither society lasted for a long time. In contrast, the Maya waged aristocratic wars, were more conservative, avoided having large standing armies of commoners, and as a result were less expansionistic and in the long run more stable. I believe that we can include the Olmec with the Maya in these characteristics. Additionally, I believe that the Zapotecs more closely resemble the Maya than the Basin of Mexico states. (page 295)
The existence of a standing army as described in the book of Alma would have been highly unusual in ancient Mesoamerica, and would have required a highly urbanized and centralized polity, which did not exist in ancient Mesoamerica during the specified time period.
A related problem can be demonstrated in how these Book of Mormon standing armies were maintained.
13 And now these are the cities of which the Lamanites have obtained possession by the shedding of the blood of so many of our valiant men;
14 The land of Manti, or the city of Manti, and the city of Zeezrom, and the city of Cumeni, and the city of Antiparah.
15 And these are the cities which they possessed when I arrived at the city of Judea; and I found Antipus and his men toiling with their might to fortify the city.
26 And thus, with their forces, they were determined to maintain those cities which they had taken.
27 And now it came to pass in the second month of this year, there was brought unto us many provisions from the fathers of those my two thousand sons.
28 And also there were sent two thousand men unto us from the land of Zarahemla. And thus we were prepared with ten thousand men, and provisions for them, and also for their wives and their children.
29 And the Lamanites, thus seeing our forces increase daily, and provisions arrive for our support, they began to be fearful, and began to sally forth, if it were possible to put an end to our receiving provisions and strength.
Alma 56, BC 65
The fathers of these sons would have been from Jershon, which had been earlier given to the Lamanite converts. Jershon was approximately 160 miles from Zarahemla, and Judea, where the army was stationed, was another 54 miles further south. They were sending provisions for a large number of people over a distance of 210 miles away. Diane and Arlen Chase, in their essay “Texts and Contexts in Maya Warfare: A Brief Consideration of Epigraphy and Archaology at Caracol, Belize”, in Ancient Mesoamerican Warfares state:
We have previously used archaeological data, ethnohistorical inference, hieroglyphic statements of aggression, warfare distance, and military theory to argue that Maya warfare could be broken into wars between primary centers and wars for border control. Border wars between primary and secondary centers are recorded as formal star-wars had an average distance of 36 kilometers between the two antagonistic centers. In contrast, the distance between primary centers who warred with each other averaged 96.5 kilometers. Both of these distances are in accord with military theory concerning marching distance and suggest that Maya polities could reasonably maintain physical and territorial borders to a radius of 60 kilometers, which translates into the distance that could be effectively marched by an army in three days. The direct territorial control implied by effective marching distance, in turn, has implications for the average size of Classic Maya polities. Following the logic of this military theory, polities that were dominated by primate centers (such as Caracol) could approach a spatial size of approximately 11,300 square kilometers. Larger-size polities could have been accommodated but likely would have been correlated with hegemonic control, more like that undertaken by the later Aztecs. (page 187)
An army could march 60 kilometers, or 37 miles, in three days. Keep in mind, however, that this was not simply a marching army, but one carrying provisions. From the book Ancient Mesoamerica: A Comparison of Change in Three Regions, Richard Blanton explains:
We need to specify more precisely why Mesoamerica’s elite found it desirable to interact, and what their exchange included. Human carriers moved raw materials and finished items of jade, crystal, metal, feathers, jaguar skins, cotton, and cacao, and the costliest pottery types. Knowledge of writing and the calendar was limited to the elite, yet was pan-Mesoamerican in distribution, as were the ball game and its paraphernalia. Certain pieces of rank-status apparel – headdresses, ear spools, nose plugs, shields – were made to regional sumptuary specifications, but they were likely to be crafted from imported materials. Furthermore, they were universally recognized as symbolic statements about the human versus the animal realm, and about the purity and degree of power of their wearers. In fact, many sacred concepts and their physical representations, rituals, and cult figures – such as the feathered serpents, lords of the underworld, rain-lightening deities (Chaco, Tlaloc, Dzahui, Cocijo), Eehcatl, and Totec – were transregional. The significance of self-sacrifice, ancestor veneration, and funeral rituals were explicitly understood among the elite across all across Mesoamerica.
The elite prestige system did not comprise all of the interregional exchange in Mesoamerica. In addition to the rich trade, some consumer goods flowed among regions. Obsidian, for example, was mined in only two main mining areas (the northern basin of Mexico and highland Guatemala), turned into cores and tools, and carried to all parts of Mesoamerica. Regional economies then saw to its distribution, which was usually differential – in some contexts restricted to the elite, in some places not. Cotton, cacao, and certain other food items may have had interregional distributions, but evidence for this prior to the fifteenth century is scarce.
Apparently the elite exchange provided the stimulus, opportunity, procedures, and established structure for a more irregular traffic in bulk commodities or items in mass demand. There is little evidence for the formation of regular interregional dependencies for goods in common use. Mesoamerica’s relatively high cost of transport undoubtedly made a macroregional-scale food economy less feasible transport undoubtedly made a macroregional-scale food economy less feasible than it was in Europe or China. To illustrate, we may consider the problem of supplying maize, a common food, to the Valley of Mexico from Tuxtepec, an important trade center on the Papaloapan River. It had to be moved by tameme, human carriers, who typically carried a load of thirty kilos and who traveled twenty-four kilometers a day, according to sixteenth century evidence. Over the most convenient trails, the journey to Mexico would take eighteen days. A person will consume, however, the equivalent of at least two-thirds of a kilo of corn per day. If we look at the food energy alone, we see that our tameme will consume 80 percent of the load over the round trip. This is not to say that bulk items never moved among regions, for other considerations beside the total energy cost may have at times been involved – for example, the providers, not the consumers, may have sometimes paid the cost of transport, as in the Aztec tribute systems. These “noneconomic” conditions, however, place the question back in the realm of exchange determined by elite relationships. That is why the Mesoamerican rich trade is not simply an archaeologically preserved proxy for trade in basic commodities. It had a purpose of its own. There is nonetheless a more subtle connection between elite exchange and basic production.
Specific, rational goals and strategies lie behind the exchange of goods, people, and information among Mesoamerica’s regional elites. Many of the material items, such as jade or feathers, were important in the scheme of rewards. Control over these prestige items was a key aspect of ruler’s power, because the ability to mass and direct large numbers of people was the measure of greatness. In Mesoamerica, people were the major factor in the production of wealth. More followers meant that more food could be produced, more craft items could be manufactured, and more warriors could be dispatched to the fields of battle. In the absence of coercion, followers had to be enlisted and their captains rewarded with the universally recognized badges of prestige. (page 220)
This means that the army carrying provisions would travel at an even slower rate, around 14 miles a day. It would take fifteen days to transport these provisions to Judea, during which time the transporters would have consumed the majority of the provisions. The sending of provisions to the army is mentioned several times in the Book of Mormon, and presents the same inconsistencies. However, the problem is increased with a different scenario, again including the idea of travel and provisions:
21 But Gidgiddoni saith unto them: The Lord forbid; for if we should go up against them the Lord would deliver us into their hands; therefore we will prepare ourselves in the center of our lands, and we will gather all our armies together, and we will not go against them, but we will wait till they shall come against us; therefore as the Lord liveth, if we do this he will deliver them into our hands.
22 And it came to pass in the seventeenth year, in the latter end of the year, the proclamation of Lachoneus had gone forth throughout all the face of the land, and they had taken their horses, and their chariots, and their cattle, and all their flocks, and their herds, and their grain, and all their substance, and did march forth by thousands and by tens of thousands, until they had all gone forth to the place which had been appointed that they should gather themselves together, to defend themselves against their enemies.
23 And the land which was appointed was the land of Zarahemla, and the land which was between the land Zarahemla and the land Bountiful, yea, to the line which was between the land Bountiful and the land Desolation.
24 And there were a great many thousand people who were called Nephites, who did gather themselves together in this land. Now Lachoneus did cause that they should gather themselves together in the land southward, because of the great curse which was upon the land northward.
25 And they did fortify themselves against their enemies; and they did dwell in one land, and in one body, and they did fear the words which had been spoken by Lachoneus, insomuch that they did repent of all their sins; and they did put up their prayers unto the Lord their God, that he would deliver them in the time that their enemies should come down against them to battle.
3 Nephi 3, AD 17
Once again, the Nephite region would have covered 75,000 square miles. People from throughout this region were traveling, carrying provisions with them. Yet only those from very nearby would not consume a majority of the provisions en route. With so many people living in a much smaller region than normally, it would have been very difficult to provide for all their needs, so they certainly would have needed those provisions. Yet, at the end of this seven or nine year period (3 Nephi 4:4), the Nephites still had plenty of provisions left over!
1 And now it came to pass that the people of the Nephites did all return to their own lands in the twenty and sixth year, every man, with his family, his flocks and his herds, his horses and his cattle, and all things whatsoever did belong unto them.
2 And it came to pass that they had not eaten up all their provisions; therefore they did take with them all that they had not devoured, of all their grain of every kind, and their gold, and their silver, and all their precious things, and they did return to their own lands and their possessions, both on the north and on the south, both on the land northward and on the land southward.
3 Nephi 6, AD 26
The probability of this event having taken place in ancient Mesoamerica, with the limitations of transportation and political control, is astonishingly small. It is so small that I feel comfortable stating that there is simply no way this event could have occurred in ancient Mesoamerica.
If this event occurred in ancient Mesoamerica, then it required the existence of a highly urbanized, centralized polity with a far more efficient transportation system than actually existed in ancient Mesoamerica, and that same polity and transportation system disappeared without a trace.
If these events occurred in ancient Mesoamerica, then it requires that two highly urbanized, centralized polities exist during the time frame, because all the problems described for the Nephites are just as equally linked to the Lamanites. Yet both of these polities disappeared without a trace.
http://www.mormonmesoamerica.com/politiesandpower.htm