John Gill Commentary 2 Chronicles 20:17

"Ye shall not need to fight in this [battle]: set yourselves, stand ye still, and see the salvation of Jehovah with you, O Judah and Jerusalem; fear not, nor be dismayed: to-morrow go out against them: for Jehovah is with you." — 2 Chronicles 20:17 (ASV)

You shall not need to fight in this battle Since the Lord would fight for them:

set yourselves, stand you still; present themselves they might, as if ready to engage; and keep their ground, not giving way in the least, but would have no need to strike a stroke:

and see the salvation of the Lord with you; which he would work for them, see (Exodus 14:13),

and Judah, and Jerusalem, fear not, nor be dismayed; which is repeated for the confirmation of them:

tomorrow go out against them, for the Lord will be with you; to protect them, fight for them, and give them victory; the Targum is, ``the Word of the Lord shall be your help.''