Read it HERE.
Summary: The righteous Lamanites preach to the wicked Nephites—Both peoples prosper during an era of peace and plenty—Lucifer, the author of sin, guides the Gadianton robbers in their murders and wickedness—They take over the Nephite government. Between 29 and 24 B.C.
This is an interesting chapter. I'm going to be lazy this week and copy the following list from the Institute Manual. It really lays out the characteristics of the secret combinations and I found it helpful to go through it. Feel free to skip (as it is lengthy), but if you want to read more you can find it here.
From this chapter and other places in the Book of Mormon we learn the following important characteristics of secret combinations:
1. The devil is the inspiration and source of all such organizations (see Helaman 6:26–30 ). Mormon clearly pointed to Satan as the grand conspirator, the real organizer of all such organizations.
2. Such organizations are viewed by the Lord as constituting a wickedness “above all the wickedness of the whole earth” ( 3Nephi 9:9 ; see also Ether 8:18 ). While individuals may rob, steal, plunder, and murder, how much more wickedness occurs when men plunder and kill on a national or an international scale involving millions of lives?
3. Such organizations flourish and thrive when the “more part” ( Helaman 6:21 ) of the people are wicked and seek to benefit from the spoils of such wickedness (see v.38 ).
4. Secrecy is one of the basic operating tenets of such organizations (see vv.22, 25–26 ).
5. Joined with the idea of secrecy is the idea of making covenants (see vv.22, 25–26 ). An oath of loyalty to the organization is made which involves a vow to maintain the secret of the conspiracy. After the murder of the chief judge, Kishkumen “went unto those that sent him, and they all entered into a covenant, yea, swearing by their everlasting Maker, that they would tell no man” ( Helaman 1:11 ). Helaman 6:21 refers to “their covenants and their oaths.” Moroni described this same covenant making among the Jaredites (see Ether 8:13–14 ). Ironically, the oaths were sometimes made in the name of God (see Helaman 1:11 , Ether 8:14 ).
6. The objectives of such secret combinations are power or gain, or both (see Helaman 6:38 , Ether 8:22 , Moses 5:31–33 ). Since the government is a source of great power, it is not surprising that often the target of action is to take over the reins of government (see Helaman 1:1–4 , 2:4–5 , 3Nephi 3:6 , 6:30 , 7:12 , Ether 9:1–6 , 13:18 ).
7. These organizations use immorality, money, and violence to achieve their ends (see Helaman 6:15, 17 , Ether 8:10 ). Assassinating government leaders to bring their own people to power is a common tale found in the Book of Mormon.
8. The only way to lastingly curtail these organizations once they are established and begin to flourish is through conversion of the people to righteousness (see Helaman 6:37 , 3Nephi 5:4–6 ).
Now, how is it that what started as a small group of wicked, plotting people became a large group known as the Gadianton Robbers? Verse seventeen states that the Nephites began to set their hearts upon riches. Once a people starts down this path of "idol" worship, they begin to commit sins in order to procure more riches. Then the sins don't seem to be a big deal anymore, because the riches are more important to them than God. It was a quick road to wickedness for the Nephites.
The Lamanites, however, grew stronger in righteousness and faith. When the Gadianton robbers began to be a real problem, what did they do? They did not go to war in the usual sense, but they fought the wickedness at its root:
37 And it came to pass that the Lamanites did hunt the band of robbers of Gadianton; and they did preach the word of God among the more wicked part of them, insomuch that this band of robbers was utterly destroyed from among the Lamanites.
And the Nephites? Well, the Nephites "built them up and support[ed] them [. . .] until they had overspread all the land." (vs. 38)
"Where your treasure is, there will your heart be also" (Matt. 6:21)