2 Chronicles 20 also 1 Kings 22:48-49

What is this passage all about?
Judah was threatened by the Moabites and Ammonites from Edom. In the assembly, Jehoshaphat prayed to God for help and a Levite prophesied that God would deliver them. All Judah praised God through song and went to see this deliverance; the invading armies attacked each other and Judah was able to collect the plunder before praising God publicly again. Much later, Jehoshaphat built a fleet of trading ships but they were wrecked before they could set sail.

What can I learn from it?
This passage tells a story of a king's plea to God and the people's praise to God. When Jehoshaphat was told that there was an invading army on the way, he was alarmed, but he did the right thing. He proclaimed a fast for all Judah - a time for the people to contemplate their sin and to pray to God. Then (v4), "[t]he people of Judah came together to seek help from the LORD; indeed they came from every town in Judah to seek him." It is remarkable that there was such a quick response from all the people of Judah. I think Jehoshaphat's reforms in ensuring the people were taught well from the book of the law (2Ch17) and judged well according to God's justice (2Ch19) meant that the people were united in their faith. As Jehoshaphat prayed before the assembled people of Judah and Jerusalem in the temple (v13), "All the men of Judah, with their wives and children and little ones, stood there before the LORD." They were expecting God to protect them from their enemies, because they knew that He was their God, and He had chosen them as His people and given them the land where they lived.
Jehoshaphat prayed in praise of God (v6): " 'O LORD , God of our fathers, are you not the God who is in heaven? You rule over all the kingdoms of the nations. Power and might are in your hand, and no one can withstand you.' " He also spoke to God of the time when His people were entering the promised land and God had directed them to leave certain neighbours unmolested as they travelled past their lands. Then Jehoshaphat called upon God to judge these nations which were invading the land God had promised to Israel. Jehoshaphat acknowledged Judah's powerlessness (v12): " 'We do not know what to do, but our eyes are upon you.' "
God responded to this honest and direct plea for grace. He sent a message through Jahaziel, a Levite, that He would take care of things (v17). " 'You will not have to fight this battle. Take up your positions; stand firm and see the deliverance the LORD will give you, O Judah and Jerusalem. Do not be afraid; do not de discouraged. Go out to face them tomorrow, and the LORD will be with you.' "
Jehoshaphat was not discouraged! On the contrary, he and all Judah were celebrating even as they left for the Desert of Tekoa. The second half of this passage is full of worship. Jehoshaphat and the people bowed and worshipped God (v18), Jehoshaphat appointed people to sing God's praises as they went out (v21), as they sang God confounded the opposing armies and they turned on each other (vv22-23). After they took the spoils of war, the people "praised the LORD" at what was called thereafter the Valley of Praise (v26). The men returned to Jerusalem joyfully "for the LORD had given them cause to rejoice" (v27) and they went to the temple to praise God with music.

How does this help me worship God?
I have heard it argued by some people that praising God through song is not a legitimate form of worship for "these last days" (using Romans 12:1 as the basis for the argument). Of course this passage describes a situation from OT times, but I do not see how Christ's life, death and resurrection have changed anything about how Christians should praise God compared to this example. Jehoshaphat had a problem, he brought it to God and God gave him a great solution. So Jehoshaphat praised God (and led the people of Judah in praise) in word and in song (vv21-22). "Jehoshaphat appointed men to sing to the LORD and to praise him for the splendor of his holiness aas they went out at the head of the army, saying: 'Give thanks to the LORD, for his love endures forever.' As they began to sing and praise, the LORD set ambushes..." God loves to hear His people praise His name and sing of His glory. Sure, there are a lot of modern "worship" songs which don't praise God at all. But that doesn't mean that Christian congregations cannot be discerning in the choice of songs and then sing all out with gusto to the great and marvellous God who saved Jehoshaphat's people and has also saved all His chosen children. And feel comfortable enjoying singing to the LORD of all.