Jamieson, Fausset and Brown on 2 Chronicles 28:16

16. At that time did king Ahaz send unto the kings of Assyria--"kings," the plural for the singular, which is found in many ancient versions. "At that time," refers to the period of Ahaz' great distress, when, after a succession of defeats, he retreated within the walls of Jerusalem. Either in the same or a subsequent campaign, the Syrian and Israelitish allies marched there to besiege him (see on 2Ki 16:7). Though delivered from this danger, other enemies infested his dominions both on the south and the west.