Joab gives a harsh but needed rebuke to David (after defying his orders in the last chapter!).
David sends a peace offering to Judah and Absalom's military commander Amasa, that he's willing to accept their invitation back to the throne peaceably. They do it. Shimei grovels, Abishai wants to kill him again, and David pardons him. Mephibosheth says Ziba slandered him; David, foreshadowing Solomon with the two mothers, divides Mephibosheth's estate between them, and Mephibosheth shows his desire for David.
Israel and Judah dispute over who supports David more. It gets so sharp Sheba calls for a rejection of David, again! David tells Amasa to muster the army. He takes too long, and David gives the job to Abishai and Joab again. They meet and kill Amasa on the road. Pursuing Sheba, they beseige a city. A wise woman there stops the violence by having the city leaders kill Sheba.
How this is about Jesus / Application
The church is fickle, sometimes zealous for the Lord, other times willing to abandon Him for other would-be leaders.
The church will always have its Joabs, who are willing to take the reins and steer the ship of state where they want according to worldly wisdom, regardless of Christ's orders. This leads to brother harming brother, which rightly disturbs and distracts the rest from carrying out their callings.
Mercy is better than vengeance and justice (Shimei).
Earthly justice is imperfect (Ziba).