In this chapter, Abner defects to David's side, but is murdered by Joab.
This is another fascinating chapter that explores a lot of the political dynamics that I touched on in the previous chapter.
First of all, this chapter begins with "a long war", and though it doesn't tell us how long the war is, it does tell us that David has six sons by six different wives, so it had to have been a minimum of one year, but more likely it is somewhere between 3 and 6 years.
One interesting little detail from v. 3 is that Absalom's grandfather is the king of Geshur. Geshur is one of the native tribes that inhabited Canaan before the Israelites moved in. They are referenced in Joshua 13 as being one of the tribes that was not driven out by Israel, and referenced again in 1 Samuel 27 as being one of the tribes that David himself would attack while he was living in Gath. These were the people that Moses commanded the Israelites that they should not intermarry with.
So at least to some extent, David marrying Maacah is contrary to the Law of Moses. Without trying to justify David's action, I'll just point out that it was very common in antiquity for the royalty of different nations to intermarry in order to form political alliances or end wars. I think this is plausibly true for this case, that David married Maacah in order to end conflict between Judah and the Geshurites.
Next, we are told that David is growing progressively stronger while "the house of Saul", i.e. Ish-Bosheth and Abner, is growing progressively weaker.
Verse 6 tells us that "Abner was making himself strong in the house of Saul." To those who read my commentary over the last 2 chapters, this should not be a surprise because I already stated that Abner was the person who "made Ish-Bosheth king", and therefore is already in a position where he could very well take away Ish-Bosheth's kingdom. Abner is in a strong position "in the house of Saul", but "the house of Saul" is progressively losing ground to the "house of David". Although it's hard to discern Abner's intentions at this point, it is clear that his position is growing increasingly tenuous as David wins more victories, so Abner is perhaps making some sort of move to establish some kingdom of his own.
In v. 7, Ish-Bosheth accuses Abner of having sex with his father's concubine. The text does not exactly say if this is true, although v. 6 seems to imply that Abner might have done so. Something like this happened before when Reuben had sex with his father Jacob's concubine, in Genesis 35. Jacob got so upset about it that he gives the double portion (which would normally have gone to the firstborn) to his favored son Joseph, and that's why the tribes of Ephraim and Manasseh get equal portions with the other sons of Jacob.
In this case, Abner is probably making a similar calculation, trying to sleep with Saul's concubine in order to establish himself as Saul's successor. Ish-Bosheth obviously senses Abner trying to maneuver around him, but Ish-Bosheth has little leverage over Abner, and simply accuses him of the act without having any real ability to move against Abner. Not only does Abner know that Ish-Bosheth can't touch him, Ish-Bosheth knows it too, as v. 11 makes abundantly clear. This proves to be a fatal mistake for Ish-Bosheth, because Abner already sees the house of Saul circling around the drain (so to speak), and takes this as his opportunity to defect to David.
David, for his part, has not forgotten about his wife Michal, the daughter of Saul, whom he married back in 1 Samuel 18. Michal had been loyal to David and loved him, but was separated from him when David fled Gibeah, and it appears that David has remained attached to Michal even though he now has at least 6 wives or concubines. What makes this story a little weird is that David gives this as a condition to Abner (that Abner should send Michal), but then in v. 14 it says that David sent messengers to Ish-Bosheth, and that Ish-Bosheth sends Michal to him. Abner's incentive is clear, but what I don't understand is why Ish-Bosheth is motivated to send Michal to David. Common courtesy, perhaps? But these situations are so political because Michal is a daughter of Saul (and therefore marries David into the house of Saul), so it just seems like there had to have been some important subtleties about this. I would guess that Abner pressures Ish-Bosheth into releasing Michal to David.
To summarize, Ish-Bosheth and Abner are tied together in a fragile alliance. Abner decides to betray Ish-Bosheth, and David asks for Michal in exchange for sparing Abner's life. Abner (probably) pressures Ish-Bosheth to release Michal to David (the text does not directly say this, but I think it's the most likely explanation), but it is unlikely that Ish-Bosheth realizes that Abner is already betraying him.
V. 16 is particularly striking, because it shows us how Paltiel is also emotionally attached to Michal (Paltiel is the man to whom Michal was given after David fled). However, Abner is direct and ruthless, and Paltiel is sent home.
After sending Michal to David, Abner visits the elders of Israel to convince them to join David, and then visits David to relay Israel's acceptance of David as their new king. I think it's funny how Abner starts talking about "the LORD has spoken of David" after he is done negotiating his own assurances and after serving Saul and Ish-Bosheth for years. It's not like Abner heard these prophecies for the first time immediately before defecting. So Abner referring to the LORD is pretty cynical right now, in my opinion.
Regardless, Abner makes peace with David and goes off to draw together all the people of Israel to come and anoint David as king. Joab happened to have been away at the time, and when he gets back, he immediately warns David that Abner came to spy on him, but in truth Joab is thinking about his dead brother, Asahel, who was slain by Abner. Joab sends for Abner again, not content to let him live, and murders him deceptively.
What follows after this is also quite interesting. David mourns the death of Abner, and disclaims any responsibility for his murder, but does not act against Joab. David pronounces a curse upon Joab, but cannot act against him, for many of the same reasons that Ish-Bosheth could not act against Abner. Now, Joab did not "make David king", but Joab was a very powerful soldier and very influential as the commander of David's armies, and David himself is only now taking Israel as his kingdom. In v. 39, David says that he is "weak today", and that weakness is ultimately why he cannot act against Joab, even though Joab killed an innocent person against his will.
This is how the death of Asahel has repercussions throughout later parts of this story, because after Joab kills Abner, it sours the relationship between David and Joab, and ultimately David never forgets it, nor does he ever forgive Joab, but he never has the power or courage to move against Joab during his lifetime.