Notice first how the Lord encourages His children:
And he said, Hearken ye, all Judah, and ye inhabitants of Jerusalem, and thou king Jehoshaphat, Thus saith the LORD unto you, Be not afraid nor dismayed by reason of this great multitude; for the battle [is] not yours, but God's. (verse 15)
Ye shall not [need] to fight in this [battle]: set yourselves, stand ye [still], and see the salvation of the LORD with you, O Judah and Jerusalem: fear not, nor be dismayed; to morrow go out against them: for the LORD [will be] with you. (verse 17)
Indeed, He knows our frame! He is actually the One who is fighting for us in every battle with sin. Only by His strength we can gain victories.
Next, something very interesting is happening:
And Jehoshaphat bowed his head with [his] face to the ground: and all Judah and the inhabitants of Jerusalem fell before the LORD, worshipping the LORD. And the Levites, of the children of the Kohathites, and of the children of the Korhites, stood up to praise the LORD God of Israel with a loud voice on high. (vs. 18-19)
They started praising the Lord even before they went into battle! The same way, we should not only ask the Lord for the victory, but at the same time we should thank and praise Him for it and claim it for ourselves!
See what followed in this story:
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 the LORD your God, so shall ye be established; believe his prophets, so shall ye prosper.
And when he had consulted with the people, he appointed singers unto the LORD, and that should praise the beauty of holiness, as they went out before the army, and to say, Praise the LORD; for his mercy [endureth] for ever.
And when they began to sing and to praise, the LORD set ambushments against the children of Ammon, Moab, and mount Seir, which were come against Judah; and they were smitten. (vs. 20-22)
Singing and praising the Lord, they went into the battle, and He acted on their behalf!
I found this a very powerful lesson. Never should we wait to claim the Lord's victory on our behalf until we are actually tempted. We can strengthen and increase our faith by not only asking, but claiming victories in advance - and then trust that the battle is indeed not ours, but the Lord's!