John Gill Commentary 2 Samuel 10:7

"And when David heard of it, he sent Joab, and all the host of the mighty men." — 2 Samuel 10:7 (ASV)

And when David heard of [it]
This refers to the preparation made by the Ammonites to fight with him:

he sent Joab and all the host of the mighty men :
He sent out Joab his general, and an army under his command, consisting of men of strength, valour, and courage; or all the host and the mighty men, as Kimchi and Ben Melech, the famous mighty men mentioned in (2 Samuel 23:8). He did not think it advisable to wait for the Ammonites, but carried the war into their own country, and, instead of suffering them to invade his dominions, he invaded theirs.