Charles Spurgeon Commentary 2 Chronicles 20:20-21

"And they rose early in the morning, and went forth into the wilderness of Tekoa: and as they went forth, Jehoshaphat stood and said, Hear me, O Judah, and ye inhabitants of Jerusalem: believe in Jehovah your God, so shall ye be established; believe his prophets, so shall ye prosper. And when he had taken counsel with the people, he appointed them that should sing unto Jehovah, and give praise in holy array, as they went out before the army, and say, Give thanks unto Jehovah; for his lovingkindness [endureth] for ever." — 2 Chronicles 20:20-21 (ASV)

So you can see them marching out of the city gate with the king at their head, and, as they go out, the army is marching with banners and with songs and hosannas. This is their style of going out to meet the foe.