Albert Barnes Commentary 2 Kings 16:5

"Then Rezin king of Syria and Pekah son of Remaliah king of Israel came up to Jerusalem to war: and they besieged Ahaz, but could not overcome him." — 2 Kings 16:5 (ASV)

Rezin and Pekah, who had already begun their attacks upon Judea in the reign of Jotham (2 Kings 15:37), regarded the accession of a boy-king, only 16 years of age, as especially favorable to their projects and proceeded without loss of time to carry them out. The earlier scenes of the war, omitted by the writer of Kings, are given at some length in 2 Chronicles 28:5–15.