I thought this verse was an excellent example of "playing both ends against the middle," as defined at
http://dictionary.reference.com/browse/play%20both%20ends%20against%20the%20middle
(- see definition 77 there.)

It's also an example of God's grace in answering a prayer in an overwhelming way.

In this chapter, Jehoshaphat made a 180 degree switch from saying 'We don't know what to do" to "Let's march forward and trust God." And God not only helped them defeat their enemies - when they arrived at the ambush point the enemies had defeated each other! There was no battle left for the Israelites to fight.

2 Chronicles 20:23