The Nephites had a cycle of righteousness and peace to riches and pride, to wickedness and war, to repentance and back to righteousness (see Alma 4:5–13; Helaman 3:32, 4:12). The Jaredites had a similar cycle, as illustrated in these verses. They went from a blessed people to the oaths of the secret combinations, to prophets warning of the land being cursed and destroyed, to the kingdom being overthrown, to captivity, to overthrowing the wicked king, and back to prosperity. We see three of these cycles in Ether 11, (11:1–8; 11:11–18; and 11:19–23). The cycle was not always recorded completely, but there are elements of the cycle in all three time periods. Remember that Moroni is abridging and giving an overview in many cases. The prophets in this cycle again warned of utter destruction (v. 12).
There are three generations in the above verses. In the genealogy of Jared, Ethem is listed as a son of Ahah. Therefore, there is only one generation from Ahah to Ethem. The third generation is Moron, (v. 16). The unnamed mighty man was a descendant of the brother of Jared, and he overthrow Moron and his kingdom (vv. 17–18). This overthrow was still in the same generation.