Jamieson, Fausset and Brown on 1 Chronicles 19:6

6. when the children of Ammon saw that they had made themselves odious to David--One universal feeling of indignation was roused throughout Israel, and all classes supported the king in his determination to avenge this unprovoked insult on the Hebrew nation.

Hanun ... sent a thousand talents of silver--a sum equal to £342,100, to procure the services of foreign mercenaries.

chariots and horsemen out of Mesopotamia ... Syria-maachah, and ... Zobah--The Mesopotamian troops did not arrive during this campaign (1Ch 19:16). Syria-maachah lay on the north of the possessions of the trans-jordanic Israelites, near Gilead.

Zobah--(see on 1Ch 18:3).