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.