When David went to fight against the Philistines “he became exhausted.” Was it because he was fighting in his own power and not depending on God?

There is an absence of his inquiring of the Lord before going into battle. His men suggested that he stay out of the battle. So in the spring “when kings go off to war” David stayed (KJV says “tarried”) in Jerusalem. Bad idea! It was because David “tarried” that he had time on his hands to give in to his wandering eye and to lust after Bathsheba. And if the lusting weren’t enough, he sent for her, slept with her and she conceived. After finding out that Bathsheba was pregnant, David sent for her husband Uriah in hopes that he would sleep with her and that the baby would be mistaken for Uriah’s. This would let David off the hook. However it didn’t work. David sunk to an unprecedented low when he ordered that Uriah be sent to the front line of battle. At the death of Uriah, David was then free to marry Bathsheba.

Using the parable of the "one little ewe lamb," the prophet Nathan confronted David ("You are the man!") with his double sin, the murder of Uriah the Hittite and his adultery with Bathsheba, Uriah's wife (2 Sam. 12:1-15) David in effect pronounced his own sentence when he responded to the prophet Nathan’s story about the poor man and his ewe lamb. David's reign was one of constant battle with his enemies.

And yet, David is known as a man after God’s own heart. What grace! I believe it’s called Amazing Grace.