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?

In 2 Samuel 21:18-22 are listed several names and descriptions of battles with the Philistines.  There is mention of a Bethlehemite who killed Goliath. This man was David who was from Bethlehem.  Notice that Goliath had a least four brothers. This may be why David selected five stones for his slingshot when he fought the giant Goliath. (1Samuel 17:40) 

There is an absence of David 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.

Uriah was a man of honor.  He was devoted to his troops and would not indulge himself by going home to his wife when his men were not able to do so.  Even in a drunken stupor, Uriah maintained his integrity.

David was given authority and absolute power over everything possessed by Saul, even to Saul's wives and concubines.  Taking the whole harem of King Saul was sure proof that David was king.  Most of David's wives may have been from Saul's harem. This may be why God told Nathan the prophet that He (God) had given the master's wives into David's arms.

Through Psalm 51 David repents of his sin with Bathsheba.
"Surely I was sinful at birth, sinful from the time my mother conceived me."
(see John 3:17-18)

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.