Charles Ellicott Commentary 2 Chronicles 20:35

"And after this did Jehoshaphat king of Judah join himself with Ahaziah king of Israel; the same did very wickedly:" — 2 Chronicles 20:35 (ASV)

And after this. —The chronicler has omitted the notice that Jehoshaphat made peace with the king of Israel (1 Kings 22:44), and now he omits two other short verses of the parallel account, namely, 1 Kings 22:46–47: And the remnant of the sodomites, which had remained in the days of his father Asa, he consumed out of the land. There was then no king in Edom: a deputy was king. The former omission is perfectly natural, as the Qĕdçshîm were not mentioned in Asa’s reign (Compare to 1 Kings 15:12); and the latter is probably due to the fact that it was the religious aspect, and not the political antecedents, of Jehoshaphat's conduct that most interested the chronicler.

Hence also the didactic tone of the following verses as compared with 1 Kings 22:48–49. The expression “after this” can only mean after the overthrow of the three nations (2 Chronicles 20:1–30). Since Ahaziah began to reign in the seventeenth year of Jehoshaphat and reigned two years (1 Kings 22:51), the league between them was formed in the seventeenth or eighteenth year of the king of Judah.

Join himself (‘eth c habbar). —An Aramaism (here only). This verse is peculiar to the chronicle.

Who did very wickedly. —He (namely, Ahaziah, the pronoun is emphatic) did very wickedly. The implied thought is: And, therefore, Jehoshaphat’s alliance was wrong (Compare to 2 Chronicles 19:2).