John Gill's  Exposition of the Bible 

2 Chronicles 13:15

Then the men of Judah gave a shout: and as the men of Judah shouted, it came to pass, that God smote Jeroboam and all Israel before Abijah and Judah.

Then the men of Judah gave a shout,.... Taking heart at the sound of the trumpets, and in order to encourage one another, and intimidate the enemy; See Gill on "1 Samuel 17:20",

and as the men of Judah shouted, it came to pass, that God smote Jeroboam and all Israel before Abijah and Judah; possessed them with a panic, so that they fled at once, as follows.