I've been reading the "Lion of War" series by Cliff Graham. Almost finished the second book, Covenant of War. God has taught me so much through these books. They're very realistic and violent, but it will give you a good picture of these stories of David and his mighty men.
The author shares the story of Eleazar, David's number two mighty man, who was fighting the Philistines until his hand froze on the sword. In the story the Philistines are coming up through the valley of Elah and David and Eleazar are all that stands in their way. Before the battle, Eleazar asked David for an encouraging word. David says "Don't be afraid. Fear comes when we see only the enemies before us and not the beheaded champions behind us."
We all need to hear encouragement like that. How many times has God delivered our enemies into our hands, or brought us through difficult and often seemingly impossible situations? We need to focus on Him and what He has done and will do again instead of the problems looming before us. Yahweh will cover us in the day of war.