The soldiers had come to Yehudah. Why does it say "mi'Shomron v'Ad Beis Choron"? It should say oppositely!
'Rashi': After they returned to Shomron, they returned to ravage cities of Yehudah.
After Hashem helped Yehudah, why did they suffer at the hands of Yisrael?
Yalkut Shimoni (Melachim II, 234, partially cited in 'Rashi'): "Ish Chacham Nishpat Es Ish Evil v'Ragaz v'Sachak v'Ein Nachas" (Mishlei 29:9). Rebuking a fool does not help, whether one shows anger or a pleasant face. I (Hashem) was angry at Achav, and handed him over to Malchei Damesek, and he offered to their gods! I showed a pleasant face to Amatzyah, and handed over Malchei Aram to him, and he brought their gods and offered to them!
Malbim: This was a punishment for throwing the captives from the rock.
