John Gill Commentary 2 Chronicles 12:6

"Then the princes of Israel and the king humbled themselves; and they said, Jehovah is righteous." — 2 Chronicles 12:6 (ASV)

Whereupon the princes of Israel and the king humbled
themselves
Both by words acknowledging their sins, and the justice of God, and by deeds, perhaps putting on sackcloth, as was usual on such occasions, and betaking themselves to fasting and prayer:

and they said, the Lord is righteous ;
in giving them up into the hand of their enemies, seeing they had forsaken him, and sinned against him.