So Amasa went out to notify the troops, but it took him longer than the three days he had been given.
Then David said to Abishai, "That troublemaker Sheba is going to hurt us more than Absalom did. Quick, take my troops and chase after him before he gets into a fortified city where we can't reach him."
So Abishai and Joab set out after Sheba with an elite guard from Joab's army and the king's own bodyguard.
References for 2 Samuel 20:7
As they arrived at the great stone in Gibeon, Amasa met them, coming from the opposite direction. Joab was wearing his uniform with a dagger strapped to his belt. As he stepped forward to greet Amasa, he secretly slipped the dagger from its sheath.
"How are you, my cousin?" Joab said and took him by the beard with his right hand as though to kiss him.
Amasa didn't notice the dagger in his left hand, and Joab stabbed him in the stomach with it so that his insides gushed out onto the ground. Joab did not need to strike again, and Amasa soon died. Joab and his brother Abishai left him lying there and continued after Sheba.
One of Joab's young officers shouted to Amasa's troops, "If you are for Joab and David, come and follow Joab."
But Amasa lay in his blood in the middle of the road, and Joab's officer saw that a crowd was gathering around to stare at him. So he pulled him off the road into a field and threw a cloak over him.
With Amasa's body out of the way, everyone went on with Joab to capture Sheba.
Meanwhile, Sheba had traveled across Israel to mobilize his own clan of Bicri at the city of Abel-beth-maacah.
When Joab's forces arrived, they attacked Abel-beth-maacah and built a ramp against the city wall and began battering it down.