John Gill Commentary 2 Chronicles 13:12

"And, behold, God is with us at our head, and his priests with the trumpets of alarm to sound an alarm against you. O children of Israel, fight ye not against Jehovah, the God of your fathers; for ye shall not prosper." — 2 Chronicles 13:12 (ASV)

And, behold, God himself is with us for our Captain
To go before our armies, and fight our battles for us:

and his priests with sounding trumpets to cry alarm against you ;
which was one use of the trumpets, that the people might be remembered by the Lord, and saved from their enemies, (Numbers 10:9) , so that this circumstance was against Jeroboam and his army, and for Abijah and his:

O children of Israel, fight you not against the Lord God of your
fathers ;
for fighting against his people, that retained the pure worship of him, was fighting against him:

for you shall not prosper ;
he seems to be assured of victory.