Albert Barnes Commentary 2 Kings 16:9

"And the king of Assyria hearkened unto him; and the king of Assyria went up against Damascus, and took it, and carried [the people of] it captive to Kir, and slew Rezin." — 2 Kings 16:9 (ASV)

The submission of Judah, which Ahaz offered, was of the utmost importance for any plans of Egyptian conquest. Naturally, Damascus was the first target of attack. It was the head of the confederacy and lay nearest to an army descending on Lower Syria—as all Asiatic armies would—from the north.

According to one of Tiglath-pileser's inscriptions, Rezin met him in battle, was defeated, and was slain. An attack on Pekah followed. It was probably at this time that the entire trans-Jordanic region was overrun, and the Reubenites, the Gadites, and the half-tribe of Manasseh were carried into captivity (1 Chronicles 5:26). Megiddo and Dor also appear to have been occupied, and the Arabs of the south were punished.

Tiglath-pileser then returned to Damascus, where a son of Rezin had taken the crown. He besieged and captured the city, punishing Rezin’s son with death. One of his inscriptions indicates that he held a court at Damascus, where it is probable that the tributary kings of the region were summoned to pay their tributes and do homage for their kingdoms. Among the tributes brought to him at this time, those from Judah, Edom, Ammon, Moab, Gaza, Ashkelon, and Tyre are mentioned.

Kir - Kir is mentioned by Amos as the country from which the Syrians came (Amos 9:7). Isaiah joins it with Elam, or Elymais (Isaiah 22:6). Its position can only be conjectured. Perhaps the word designates a region next to Elymais, in the far southeastern limits of Assyria.