Biblical Commentary on the Old Testament, by Carl Friedrich Keil and Franz Delitzsh, [1857-78], at sacred-texts.com
2 Chronicles 27:1
Jotham having ascended the throne at the age of twenty-five, reigned altogether in the spirit and power of his father, with the single limitation that he did not go into the sanctuary of Jahve (cf. Ch2 26:16.). This remark is not found in 2 Kings 15, because there Uzziah's intrusion into the temple is also omitted. The people still did corruptly (cf. Ch2 26:16). This refers, indeed, to the continuation of the worship in the high places, but hints also at the deep moral corruption which the prophets of that time censure (cf. especially Isa 2:5., Ch2 5:7.; Mic 1:5; Mic 2:1.).
2 Chronicles 27:3
He built the upper gate of the house of Jahve, i.e., the northern gate of the inner or upper court (see on Kg2 15:35); the only work of his reign which is mentioned in the book of Kings. But besides this, he continued the fortifying of Jerusalem, which his father had commenced; building much at the wall of the Ophel. העפל was the name of the southern slope of the temple mountain (see on Ch2 33:14); the wall of Ophel is consequently the wall connecting Zion with the temple mountain, at which Uzziah had already built (see on Ch2 26:9). He likewise carried on his father's buildings for the protection of the herds (Ch2 26:10), building the cities in the mountains of Judah, and castles (בּירניּות, Ch2 17:12) and towers in the forests of the mountains of Judah (חרשׁים from חרשׁ, a thicket).
2 Chronicles 27:5
He made war upon the king of the Ammonites, and overcame them. The Ammonites had before paid tribute to Uzziah. After his death they would seem to have refused to pay this tribute; and Jotham made them again tributary by force of arms. They were compelled to pay him after their defeat, in that same year, 100 talents of silver, 10,000 cor of wheat, and a similar quantity of barley, as tribute. לו השׁיבוּ זאת: this they brought to him again, i.e., they paid him the same amount as tribute in the second and third years of their subjection also. After three years, consequently, they would seem to have again become independent, or refused the tribute, probably in the last years of Jotham, in which, according to Kg2 15:37, the Syrian king Rezin and Pekah of Israel began to make attacks upon Judah.
By all these undertakings Jotham strengthened himself, sc. in the kingdom, i.e., he attained to greater power, because he made his ways firm before Jahve, i.e., walked stedfastly before Jahve; did not incur guilt by falling away into idolatry, or by faithless infringement of the rights of the Lord (as Uzziah did by his interference with the rights of the priesthood). From the כּל־מלחמתיו in the concluding remark (Ch2 27:7) we learn that he had waged still other successful wars. The older commentators reckon among these wars, the war against Rezin and Pekah, which kings the Lord began in his days to send against Judah (see Kg2 15:37), but hardly with justice. The position of this note, which is altogether omitted in the Chronicle, at the end of the account of Jotham in Kg2 15:37, appears to hint that this war broke out only towards the end of Jotham's reign, so that he could not undertake anything important against this foe.
The repetition of the chronological statement already given in Ch2 27:1 is probably to be explained by supposing that two authorities, each of which contained this remark, were used.