כִּ֚י אִם־בֹּ֣א אַתָּ֔ה עֲשֵׂ֖ה חֲזַ֣ק לַמִּלְחָמָ֑ה יַכְשִֽׁילְךָ֤ הָֽאֱלֹהִים֙ לִפְנֵ֣י אוֹיֵ֔ב כִּ֥י יֶשׁ־כֹּ֛חַ בֵּאלֹהִ֖ים לַעְז֥וֹר וּלְהַכְשִֽׁיל׃
Ma se vuoi andare e non ti impegni mai in modo così valoroso in battaglia, Dio ti abbatterà davanti al nemico; perché Dio ha il potere di aiutare e di abbattere.'
Rashi on II Chronicles
But if you come The meaning is: If you do not believe my words, will you not try [to put effort into your military campaign?] Come, do, and strengthen yourself for war.
Rashi on II Chronicles
to help or to cause to stumble The meaning is: He has the strength to help you without the army of Ephraim and to cause you to stumble if you take them with you.