There came a voice as if it were above the cloud of darkness, saying: Repent ye, repent ye, and seek no more to destroy my servants whom I have sent unto you to declare good tidings. Helaman 5:29 5:29
In 38 BCE, a war broke out in the church.
Those who rebelled were killed or driven out of the land.
3 And it came to pass that they did endeavor to stir up the Lamanites to war against the Nephites; but behold, the Lamanites were exceedingly afraid, insomuch that they would not hearken to the words of those dissenters.
But some of the rebel Nephites escaped, joined up with the Lamanites, and tried to convince them to go to war with the Nephites. But the Lamanites were "exceedingly afraid."
4 But it came to pass in the fifty and sixth year of the reign of the judges, there were dissenters who went up from the Nephites unto the Lamanites; and they succeeded with those others in stirring them up to anger against the Nephites; and they were all that year preparing for war.
5 And in the fifty and seventh year they did come down against the Nephites to battle, and they did commence the work of death; yea, insomuch that in the fifty and eighth year of the reign of the judges they succeeded in obtaining possession of the land of Zarahemla; yea, and also all the lands, even unto the land which was near the land Bountiful.
After a few years of stirring, the rebel Nephites finally succeeded, and the Lamanites attacked, defeating the lands of Zarahelma and Bountiful.
7 And there they did fortify against the Lamanites, from the west sea, even unto the east; it being a day's journey for a Nephite, on the line which they had fortified and stationed their armies to defend their north country.
8 And thus those dissenters of the Nephites, with the help of a numerous army of the Lamanites, had obtained all the possession of the Nephites which was in the land southward. And all this was done in the fifty and eighth and ninth years of the reign of the judges.
From the west sea to the east sea was a day's journey for a Nephite. 
Along the line between the two seas, the Nephites stationed their armies to defend the north country.
While the Lamanites and the Nephite dissenters controlled the land to the south.
9 And it came to pass in the sixtieth year of the reign of the judges, Moronihah did succeed with his armies in obtaining many parts of the land; yea, they regained many cities which had fallen into the hands of the Lamanites.
Moronihah regained about half of the cities that the Lamanites had captured.
11 Now this great loss of the Nephites, and the great slaughter which was among them, would not have happened had it not been for their wickedness and their abomination which was among them; yea, and it was among those also who professed to belong to the church of God.
12 And it was because of the pride of their hearts, because of their exceeding riches, yea, it was because of their oppression to the poor, withholding their food from the hungry, withholding their clothing from the naked, and smiting their humble brethren upon the cheek, making a mock of that which was sacred, denying the spirit of prophecy and of revelation, murdering, plundering, lying, stealing, committing adultery, rising up in great contentions, and deserting away into the land of Nephi, among the Lamanites --
13 And because of this their great wickedness, and their boastings in their own strength, they were left in their own strength; therefore they did not prosper, but were afflicted and smitten, and driven before the Lamanites, until they had lost possession of almost all their lands.
But many Nephites died in the process, because of the great wickedness and abomination among the Nephites.
14But behold, Moronihah did preach many things unto the people because of their iniquity, and also Nephi and Lehi, who were the sons of Helaman, did preach many things unto the people, yea, and did prophesy many things unto them concerning their iniquities, and what should come unto them if they did not repent of their sins.
16 For when Moronihah saw that they did repent he did venture to lead them forth from place to place, and from city to city, even until they had regained the one-half of their property and the one-half of all their lands.
But then Moronihah preached many things, and the people repented, began to prosper.
19 Therefore they did abandon their design to obtain the remainder of their lands, for so numerous were the Lamanites that it became impossible for the Nephites to obtain more power over them; therefore Moronihah did employ all his armies in maintaining those parts which he had taken.
Although Moronihah and the Nephites kept trying to recapture the lands taken by the Lamanites, they finally had to give up and let the Lamanites keep half their land and possessions.
22 And that they had altered and trampled under their feet the laws of Mosiah, or that which the Lord commanded him to give unto the people; and they saw that their laws had become corrupted, and that they had become a wicked people, insomuch that they were wicked even like unto the Lamanites.
The Nephites became wicked like the Lamanites.
23 And because of their iniquity the church had begun to dwindle; and they began to disbelieve in the spirit of prophecy and in the spirit of revelation; and the judgments of God did stare them in the face.
They began to disbelieve, and the judgments of God stared them in the face.
24 And they saw that they had become weak, like unto their brethren, the Lamanites, and that the Spirit of the Lord did no more preserve them; yea, it had withdrawn from them because the Spirit of the Lord doth not dwell in unholy temples --
They became weak, like the Lamanites, because the Spirit of the Lord had withdrawn from them.
25 Therefore the Lord did cease to preserve them by his miraculous and matchless power, for they had fallen into a state of unbelief and awful wickedness; and they saw that the Lamanites were exceedingly more numerous than they, and except they should cleave unto the Lord their God, they must unavoidably perish.
26 For behold, they saw that the strength of the Lamanites was as great as their strength, even man for man. And thus had they fallen into this great transgression; yea, thus had they become weak, because of their transgression, in the space of not many years.
And they fell into a state of unbelief.