Most lessons from this passage focus on the power of praising God to turn a situation around. And it's a very true and valid lesson. But there is another lesson to gleaned from this account:
Things that appear scary, bad, negative, are not always what they seem.
The King and his people could have run away in fear, but they chose to turn to and trust in God.
And instead of fighting a war, they saw God fight the war for them AND they carried away so much loot it took them THREE days to carry it all back.
Wow, so we go from being scared of people coming to make war against a nation, to seeing them carrying away the loot of the attacking nation for 3 days long.
Who would have thought that would be the end, standing at the beginning?
Well, with God.... all things are possible.