וַיַּבְדִּילֵ֣ם אֲמַצְיָ֗הוּ לְהַגְּדוּד֙ אֲשֶׁר־בָּ֤א אֵלָיו֙ מֵֽאֶפְרַ֔יִם לָלֶ֖כֶת לִמְקוֹמָ֑ם וַיִּ֨חַר אַפָּ֤ם מְאֹד֙ בִּֽיהוּדָ֔ה וַיָּשׁ֥וּבוּ לִמְקוֹמָ֖ם בָּחֳרִי־אָֽף׃ (פ)
Then Amaziah separated them, to wit, the army that was come to him out of Ephraim, to go back home; wherefore their anger was greatly kindled against Judah, and they returned home in fierce anger.
Rashi
Amatzyoh then sent messengers, etc., “let us battle each other.” [By going] to war.3Previously, Yehoash’s men had invaded Yehudah in a surprise attack, because Amatzyohu had abruptly dismissed Yehoash’s men from going to war with him. See II Divrei Hayomim 25:10, 13. Amatzyohu now challenged Amatzyohu to confront him in battle.—Metzudas Dovid