New International Version takes this verse as parenthetical, but it seems unnecessary to do so.

In those days; that is, during the time that Jotham ruled over Judah.

The LORD began to send …: This wording implies that the attacks of Rezin and Pekah were not of short duration and that Yahweh was behind the actions they took. According to 2 Kgs 16.5, the trouble with these two enemies continued after Jotham had been replaced by his son Ahaz.

Rezin the king of Syria: It will be recalled that certain modern versions (New Revised Standard Version, Revised English Bible, New American Bible, New Jerusalem Bible) retain the transliteration “Aram” instead of translating Syria (see the comments at 1 Kgs 10.29). Rezin was the last ruler of Syria before the area became a part of the Assyrian Empire in 732 B.C. The spelling of his name in the Septuagint and Assyrian documents is “Razon” (also New Jerusalem Bible, La Bible Pléiade, Osty-Trinquet). The text suggests that he became an ally of Pekah, the king of Israel, to fight against Judah. The prophet Isaiah provides additional information about this Syrian king (Isa 7.1, 4, 8; 8.6). He is mentioned again in chapter 16 (verses 5 and 9).

The son of Remaliah: As in verse 32, this detail may be omitted if the repetition is seen as distracting in the receptor language. Compare also verses 25, 27 and 30.

Quoted with permission from Omanson, Roger L. and Ellington, John E. A Handbook on 1-2 Kings, Volume 2. (UBS Helps for Translators). New York: UBS, 2008. For this and other handbooks for translators see here .