Radak: It is about a war in the days of Yehoshafat. Bnei Se'ir, Amon and Mo'av came against him (Divrei ha'Yamim II, 20:1). Other nations were with them that were not mentioned there. It mentions the Amonim, who were mentioned in the war with Uziyah with the Aravim. They are called also Me'onim (the letters of Amon are re-arranged). Also in Shoftim (10:12) "v'Tzidonim va'Amalek u'Ma'on Lachatzu Eschem", for these nations dwell near each other. Therefore it mentions here Yishmaelim, Hagrim and Amalekim; they joined with Se'ir, Amon and Mo'av.

Malbim: It was composed in the days of Yehoshafat. Bnei Mo'av and Amon came for war with great multitudes. Yehoshafat prayed and called a fast. Yachzi'el, from Bnei Asaf, had Ru'ach Hashem amidst the congregation, and said not to fear; the war is not upon you, rather, on Elokim. See his salvation! (Divrei ha'Yamim II, 20:14-17). There it says how the enemies killed each other via the panic that Hashem cast between them.