The Ammorites and Moabites and Edomites had organized "a great multitude" seeking to destroy Judah under King Jehoshaphat. But the king and his people came together to "seek the LORD" in prayer for deliverance, and God answered. "The Spirit of the LORD" spoke through Jahaziel, assuring them that "the battle is not yours, but God's" (see 2 Chronicles 20:2, 4, 14-15).
Then the Lord sent what may have seemed a strange instruction. "Stand ye still," He said. Just watch God do it! And He did. All their enemies were constrained by the Lord to fight and destroy each other, without the Israelites having to fight at all. Similarly, at the Red Sea when everything looked hopeless, "Moses said unto the people, Fear ye not, stand still, and see the salvation of the LORD" (Exodus 14:13). So they did, and all Pharaoh's armies were overthrown in the midst of the sea.
In Isaiah's day, when Israel was tempted to call on pagan Egypt for help against pagan Assyria, God said concerning Israel's armies, "Their strength is to sit still" (Isaiah 30:7). As the ship was being buffeted in the storm, and the sailors in panic were about to flee in the lifeboat, Paul said, "Except these abide in the ship, ye cannot be saved" (Acts 27:31). So they stayed, and God spared every man.
There are times for action, of course, but the principle is this. When we have done all we can, and the situation seems hopeless, this is the time to sit still and trust God to work it out in His own good way. "Be still, and know that I am God" (Psalm 46:10). Got any rivers you think are uncrossable? God specializes in things thought impossible! HMM