In this chapter, we read the genealogy for the two and a half tribes east of the Jordan.
Reuben, Gad and half of the tribe of Manasseh settled east of the Jordan in the conquered lands of Bashan and the Amorites. We first read about this story in Numbers 32. The author of Chronicles groups these three tribes together and gives their genealogical records grouped together, which makes sense when you consider the historical connection between them. Sadly, these three tribes were most likely destroyed in the Assyrian invasion which was a couple hundred years before Chronicles was written, so this genealogy is more of a memorial to these tribes than anything else, and v. 25-26 refers to the events surrounding their exile.
Verses 1-2 explain why Judah was listed first in the genealogy: not because he had the right of the firstborn (which was given to Joseph) but because "a ruler came from him", referring to David. When Reuben "defiled his father's marriage bed" back in Genesis 35:22, Jacob took away his right as the firstborn (Gen 49:3-4) and that's why these hundreds of years later Reuben is not given a place of honor in the genealogical record.
In verses 4-6, the genealogy becomes linear again, listing the father-to-son descent from Reuben down to Beerah, who was a tribal leader over Reuben in the time that they were destroyed. When the Reubenites were exiled into Assyria, the genealogical records also ceased, which ominously marks the end of the northern tribes as a distinct people.
These tribes originally went to the Transjordan region because they had large flocks and these lands were large and well-suited for semi-nomadic shepherding. It appears that they fought wars to expand their territory during the time of Saul when Israel was more successful and entering their golden age. The Hagrites (possibly related to Hagar, a.k.a. Ishmaelites, i.e. Arabs) are a nomadic people who were probably also pastoralists with large flocks and entered into conflict with the Transjordan tribes because they wanted the same resources and land. During later times, the Transjordan tribes gradually declined until they were destroyed by the Assyrians.
By the time this genealogy was written, the victories were distant memories, and even the Assyrian exile was fading into the past. However, all of these stories are written with a sense of immediacy because much of it is copied from historical records, like v. 17 which indicates that the genealogy of Gad (and possibly also Reuben and Manasseh) was recorded during the reign of Jotham.
I think if there are two themes that I want to draw out of this chapter, they are as follows:
1) The historical connection between the genealogy and previous stories from the Pentateuch;
2) Obedience to God and the Law dictates the fate of the nation. As they follow God, they are victorious, and as they turn from God they are defeated.
In light of everything I've written above, I think the connection is very clear between this genealogy and Israel's history. Not only does this genealogy reference events from Genesis and Kings, but even the layout of this chapter is derived from the book of Numbers.
The second point is just as obvious. Both the Gadites and the Manassites were valiant warriors and powerful, but God gives them victory or defeat based on their faith or lack thereof. In verse 20, the Transjordan tribes are given victory because they "cried out" in prayer and God answered them, but later in v. 25 the Manassites are "unfaithful to the God of their fathers" so God stirred up the Assyrians to come and destroy them. What I think is most interesting about this theme is how strongly it resonates with the book of Deuteronomy, where Deut 28 links the nation's obedience or lack thereof to God's covenant to all kinds of material blessings or curses respectively. I think we find this kind of pattern in the book of Kings as well (where good kings are blessed when they draw the nation back to God and evil kings suffer in their idolatry), but this pattern in general represents a very specific kind of theology where material circumstances (i.e. wealth, famines, victory or defeat in wars) are directly responsive to the people's faithfulness towards God. By identifying similarities in this kind of theology, we can see that the author of Chronicles not only knew the stories from the Pentateuch, but he also understood the ideas of the Pentateuch, in particular Deuteronomy.
We can juxtapose this kind of thinking with the book of Job (which we have not yet read) where Job suffers or is blessed regardless of how he responds to God. In a similar vein, we could study the life of Joseph son of Jacob as being non-responsive to his faithfulness towards God. He was faithful to God, got sold as a slave by his brothers, rose up in power, was thrown in prison again, and then rose up again. His story is very dynamic and it involves numerous rises and falls from power completely independent of his faithfulness towards God (which we can reasonably assume was consistent). Joseph's life is not the kind of life that easily fits into the Deut 28 pattern where blessings and curses are the result of your obedience to God's covenant. David is another figure (principally described in the book of Samuel) who has many rises and falls from power that is (mostly) independent from his faithfulness towards God.
That said, I do think there Joseph's life fits into the Deut 28 pattern if you reinterpret the idea of "curses" in the context of Joseph's life. Joseph was never cursed by God. Though he suffered many setbacks, we can see that God actually blessed him with favor and success in nearly every situation he entered. This is a much more nuanced concept than what you get in Deut 28, but I think it is more correct. This chapter in Chronicles appears to promote the simpler notion that when the tribes sought God they were victorious, and when they sought idols they were defeated. Not to say that the Chronicler is lying to us, but I think it's interesting how he presents these ideas in a way that very much agrees with Deuteronomy vs. the lives of Joseph and David.
For instance, the book of Chronicles leaves out many of the most difficult parts of David's life. It never describes his years fleeing from Saul in the southern desert or the various revolts against him (several of which are directed by his own sons). While there are multiple ways to interpret the book of Chronicles, I think it's possible that the Chronicler is viewing David's life through a Deut 28 lens, and sees it as inappropriate to describe David being "cursed" with hardships if he is faithful towards God, and hence tries to portray him as successful and exultant in everything he does. Of course, we haven't gotten to that part of the story yet, but I would like my readers to be thinking about this theology (of blessings and curses) while they read Chronicles, since I think it is emblematic of the Chronicler's view on David's life in particular and Israel's history in general, up until their descent into the Babylonian exile.