2 Samuel 19-20: Israel and Judah

But Amasa took no heed to the sword that was in Joab's hand: so he smote him therewith in the fifth rib, and shed out his bowels to the ground, and struck him not again; and he died. So Joab and Abishai his brother pursued after Sheba the son of Bichri.2 Samuel 20:10 KJV

The rift between Israel and Judah begins.

Introduction

Eventually, God's people split into two nations: The Kingdom of Israel and the Kingdom of Judah. We see the beginning of this rift in 2 Samuel 19. The men from the tribes of the north (Israel) begin to fight with the men of Judah. They felt underrepresented in the restoration of David as king and blamed Judah for "stealing" him. Judah, on the other hand, felt they had claim over the king simply because he was their kin.
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The Rift Continues

To make matters worse, a Benjamite named Sheba declares that David is not their king and all the men of Israel begin to follow Sheba. Judah, however, clings to David. David prepares for battle (once again), giving orders to the new general, Amasa. Amasa was given Joab's position as general over the army by Absalom. When David approached Judah about reclaiming the throne, he assured Amasa he could keep the position, which causes a rift of it's own. Amasa was supposed return in 3 days, but he fails to do so—this may be why David didn't choose him as the captain in the first place. David sends the more reliable Abishai instead; we aren't told why Joab was not consulted for the job.
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Kiss of Death

Amasa finally shows up to the battle, and when he does, Joab seeks revenge. Joab greets Amasa, asking if he is alright and feigning that he would greet him a kiss. In many cultures, even today, it is not uncommon for close friends to greet each other with a kiss (on the cheek). Although it seems a little weird in American culture, we must read this in the context of the time period. then proceeds to greet him with a kiss.

But Amasa took no heed to the sword that was in Joab's hand: so he smote him therewith in the fifth rib, and shed out his bowels to the ground, and struck him not again; and he died. So Joab and Abishai his brother pursued after Sheba the son of Bichri.2 Samuel 20:10 KJV

Joab had plotted out this action beforehand. He purposefully dropped his sword so that it was not odd that he had picked it up. Assuming Joab is going to re-sheath his sword, Amasa does not pay any attention to it. Instead of putting the sword away, however, Joab uses it to kill Amasa. Most likely, Joab did not appreciate Amasa taking over his role as general. Once a soldier removes Amasa from the highway and covers him with cloth, Abishai and Joab continued in pursuit of Sheba, but they leave him to bleed to death.

A point to be made is that Joab's murderous ways have been left unchecked. When Joab killed Abner, David did nothing to punish him. David probably felt Joab was justified since Abner had killed Joab's bother. However, there was no justification in killing Amasa. Since Joab knew of David's hand in the death of Uriah (Bathsheba's husband), he probably felt David wouldn't do anything to him for murdering Amasa. (He was right).
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Sheba's Death

Sheba was camped out in a city named after Abel. However, when David's men show up in Abel, a woman begs them not to destroy the city. She pledges the city's allegiance to David, explaining that they are peaceable. After speaking to the woman, Joab agrees that he will not harm the city if they hand the traitor (Sheba) over. So, the people of the city behead Sheba and return him to Joab.

It is interesting to note how quickly the people turned on Sheba. At the beginning of the chapter it seems that everyone is in agreement with him, but suddenly, they're willing to hand him over. One might ask if the people of Abel know what Sheba was up to? It is unlikely he went to the city alone, considering he knew that David would send an army to capture (or kill) him. For the city to behead him, it seems reasonable to assume his men had to turn against him too, otherwise this chapter would discuss a battle that took place between Sheba and the residents of Abel.
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Concubines

God had already prophesied that David's wives would be taken from him, and Absalom fulfilled that. Now that Absalom was dead, who did the concubines belong to? David had never formally divorced them, but Absalom had "defiled" them. We aren't told if the women sleep with Absalom willingly or are raped, but technically, without an act of adultery (rape probably didn't count), they were still married to David. As such, he continues to provide for the them as wives, however, he does not sleep with them anymore. It is suggested that this is to refrain from confusing the royal line.^[1]I don't particularly understand, because after a year or two it should be glaringly obvious that Absalom was not the father of any of their children. It seems to me more like pride.
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References

1. Homlan Bible Publishers. Holman KJV Study Bible, pg 548-551. 2014
2. Matthew Henry. "Commentary of 2 Samuel 20". BibleStudyTools.com. 2017