2 Chr 20 describes an event in the realm of Jehoshaphat, king of Judah. The chapter opens with armies preparing for war: [T]he Moabites and Ammonites, and with them some of the Meunites, came against Jehoshaphat for battle (20.1). Seeing the great armies lining up to fight against Judah, Jehoshaphat lifts up a prayer to YHWH:
O LORD, God of our ancestors, are you not God in heaven? Do you not rule over all the kingdoms of the nations? In your hand are power and might, so that no one is able to withstand you (v. 6).
As I've had opportunity to notice before, here we see that conflict between the sovereignty of God and the evil in the universe which is a ubiquitous theme of the Old Testament record. On the one hand, Jehoshaphat affirms that YHWH is the Lord of All, that he is over the entire world and he rules it in power. But on the other hand, there is an unholy alliance forming against Judah, peoples whom the Israelites did not kill when entering into Canaan. As Jehoshaphat remarks, See now, the people of Ammon, Moab, and Mount Seir, whom you would not let Israel invade when they came from the land of Egypt, and whom they avoided and did not destroy -- they reward us by coming to drive us out of your possession that you have given us to inherit (vv. 10-11).
There is a tension, therefore. Jehoshaphat is afraid (v. 3), and his confidence in God's rule is shaken. But as always, as Levenson notes, the response to these tensions and difficulties is to invoke God's power in prayer, reminding him of accomplishments of the past: Did you not, O our God, drive out the inhabitants of this land before your people Israel, and give it forever to the descendants of your friend Abraham? (v. 7). There are also regular affirmations of God's (de jure if not de facto) ownership and mastery over the world: the Ammonites et al. are attempting to drive the Israelites out of your possession, namely the land (v. 11).
The invocation has the purpose of reminding YHWH of his feats of yore, of course, but it also serves the purpose of increasing the confidence of the one making the invocation. This is a spiritual practice in which I engage at times: I remind myself of what salvation God has accomplished for me in Christ Jesus, that I am promised life eternal and freedom from sin; this has the effect of empowering me and strengthening me to begin a new day or to start over again after sin. Likewise in a prayer such as this, the recalling of God's powerful feats performed in the past has the function of strengthening the faith of the one praying. The prayer is made more confidently and more fervently when one is reminded that YHWH is not a deus otiosus, some idle god asleep at the wheel, but the rightful and powerful ruler of the whole cosmos.
This ought to be a regular practice for us as Christians, as well. In times of grave evil, when calamity and catastrophe looms over the horizon, we ought to call out to God. In the midst of our prayers, we should remember times of deliverance and salvation, the greatest of all of course being the salvation accomplished in Jesus Christ, and we ought to plea to God to save us once more. We ought to pray that God's name be glorified and that the fear of God [come] on all the kingdoms of the countries (v. 29) when they see that God responds to our prayers!