In this chapter, several kinds rise and fall over Judah and Israel.
We are once again in the midst of the royal procession that forms the heart of the book of Kings. This is an opportunity for us to rise up above the heroic stories of Elijah and Elisha, those prophets who did so much and overcame such adversity, and now we get another chance to look at the broader societal and political trends in the promised land.
In broad terms, I see two major trends here. First is the reemergence of open warfare between Israel and Judah. This was something that had started immediately after the kingdom split during the lives of Jeroboam and Rehoboam (1 Kings 14:30), but it had ceased during the reign of Ahab, who forged an alliance with the king of Judah, Jehoshaphat. This alliance included intermarriage, which is how a king of Judah ended up marrying Athaliah, who then killed all of the royal princes in order to take power for herself (2 Kings 11:1). It was a disastrous turn of events for Judah as they were sucked into the political violence and idolatry that has been plaguing the northern kingdom since nearly the day it was founded. Now that Israel and Judah are back at war, in some ways this is ironically an improvement for Judah, because they are now less likely to be impacted by Israel's political instability.
The second major trend I see is gradual decline in the southern kingdom, Judah. This is something that has been going on for a while (for instance, the rebellion of Edom and Libnah in 2 Kings 8:20-22), and it continues in this chapter. We see there is continued idolatry (v. 4). In v. 6, Amaziah seems to be doing the right thing; he is commended by the author of Kings for obeying the Law and not putting the sons of his father's murderers to death. In v. 7, Judah seems to be improving its situation with a military victory over Edom, but it rapidly deteriorates when they are defeated by Israel (v. 12). In v. 19 Amaziah is assassinated and when you combine that with his military defeat from v. 12, we can see that Amaziah's reign started off pretty well but ended poorly, following the pattern of many other kings of Judah (such as Joash). Jerusalem gets sacked again, its wall is torn down and the temple and royal palace are pillaged.
A minor textual note is that we get a second Jeroboam in this chapter. The first Jeroboam is Jeroboam son of Nabat, the first king of Israel, the new Jeroboam is Jeroboam son of Jehoash. Throughout the bible, if you ever see it mention a Jeroboam, 99% of the time it's Jeroboam son of Nabat because he is considerably more famous and is used by the biblical author(s) as a personification of the sins of Israel, because he originally set up the golden calves at Dan and Bethel. Jeroboam son of Jehoash is a fairly inconsequential figure who is mentioned incidentally a few times but not really part of the core narrative in the same way that Jeroboam son of Nabat is. Still, having two Jeroboams around was certainly enough to confuse me when I read this for the first time.
Lastly, I think verses 24-27 are pretty interesting. It begins by telling us that Jeroboam son of Jehoash did evil and led Israel into sin, but concludes by telling us that God had mercy on Israel and helped them to restore their borders (i.e. push back the foreign nations that were encroaching upon them). Verse 25 mentions a prophet by the name of Jonah, and yes, that is the Jonah from the book of Jonah (you can tell because he has the same father as the man in Jonah 1:1).
This is a brief respite, purely from the LORD's mercy. Unfortunately, since Israel is continuing to sin, the respite will end all too soon and it will be back to oppression and poverty for Israel. I don't think it's a coincidence that God mentions his mercy for Israel in the same passage that he mentions the prophet Jonah. The prophets Elijah and Elisha also brought deliverance to Israel on several occasions. I think in these cases God is bringing mercy through his prophets in order to open the hearts of the king and the people to receive what message the prophets bring. Sometimes God brings judgment through his prophets in order to break the pride and resistance of his people when they are in idolatry, and sometimes he brings mercy in order to demonstrate his compassion and concern for them. In both cases the purpose is the same: to draw forth repentance. As we can see from the progression of evil kings in Israel, the nation simply does not repent.
But I think there is another point here too, which is that human beings have access to partner with God in the fulfillment of God's plans. In this case, it is the prophet Jonah who is working with God to bring about this deliverance. Even in the midst of a sinful generation whose hearts are far from God, God draws close everyone who seeks him and he gives us opportunities to work towards his purposes and find meaning for our own lives in that labor. I think this can be an encouragement for us because Jonah is living in the wrong place at the wrong time, in the midst of an idolatrous and divided society, oppressed by other nations. But Jonah is able to rise above that and be known as a prophet of the LORD, bringing a message of deliverance to his people and serving God's purpose for his generation. Jonah's life is a remarkable testimony to God's power in him.