2 Chronicles 21:3 - Joseph Benson’s Commentary on the Old and New Testaments

With fenced cities of Judah Over which he made them governors. This he seems to have done prudently; thus providing well for them, that they might not be tempted to envy their brother the kingdom, nor to quarrel among themselves: but “things excellently designed,” says Strigelius, from Cicero, “have often a very ill event.” This wealth and authority of theirs, made their brother first fear them, and then, through his wicked covetousness, contrive to cut them off, that he might get possession of their riches and power. Because he was the firstborn Whom Jehoshaphat supposed he ought to prefer on account of the law, recorded Deuteronomy 21:15, though otherwise, it is probable, he would not have done it, having doubtless, before this time, observed his perverse and wicked inclinations, and how much he was swayed by his idolatrous wife.