דברי הימים ב כה:ח פירוש: רש״י

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.

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.