Why does it say that he returned Damesek and Chamas? They were of Aram!
Radak: David fought Aram and struck them greatly. "Va'Yasem David Netzivim ba'Aram Damesek va'Tehi Aram l'David la'Avadim" (Shmuel II, 8:6). We can say the same about Chamas, even though no verse says that he put Netzivim there. It seems that the war was in Chamas. If David took Damesek, all the more so he took Chamas!
Why does it say "li'Yhudah b'Yisrael"?
Radak: Melech Yisrael took them and returned them to Yehudah. They belonged to Yehudah, for David took them. Malbim - from when Yo'ash overpowered Amatzyah, Yehudah was conquered under Malchei Yisrael. In Amatzyah's days when his servants conspired against him, and also after he died and Uziyah ruled, they did not cast off Yaravam's yoke until Yaravam's 27th year, like it says in Perek 15. Therefore, when he conquered Aram, he returned them to Yehudah.
Malbim: Yehudah was like a province with a king under the rule of Yisrael.
