This verse, 1 Chronicles 19:8, is part of the narrative surrounding the Ammonite king Hanun's humiliating treatment of David's messengers and the subsequent war. Let's break down what it means:

"When David heard of it..." This refers to David, the King of Israel, receiving news of the shameful actions of Hanun. The news would have included how Hanun shaved off half the beards of David's messengers and cut off their garments in the middle, near their buttocks (1 Chronicles 19:4). This was a deliberate insult and a clear act of hostility.

"...he sent Joab..." Joab was David's military commander, the chief of his army. David entrusted him with the responsibility of leading the Israelite forces in response to the Ammonite provocation.

"...with all the army of the mighty men." This phrase emphasizes the strength and quality of the Israelite army being deployed. "Mighty men" refers to David's elite warriors, skilled and battle-tested soldiers known for their courage and strength. Sending "all the army of the mighty men" indicates that David was taking the threat seriously and committing a significant force to deal with the Ammonites and their allies.

In essence, the verse means:

King David, upon learning of the severe insult inflicted by the Ammonite king Hanun, immediately dispatched his top general, Joab, along with a powerful and highly skilled army, to confront the Ammonites and address their hostile actions.

The significance of this verse in the broader context:

Sets the stage for war: It marks the beginning of a military conflict between Israel and the Ammonite kingdom and their allies.
Highlights David's response: It shows David's decisive action in defending Israel's honor and security. He doesn't ignore the insult but responds with force.
Emphasizes the importance of Joab: It confirms Joab's role as David's trusted military leader.
Illustrates the strength of David's army: The mention of "mighty men" underscores the military prowess of Israel under David's leadership.