'Battles of the Lord' in the Bible

Then Saul said to David, "Here's my oldest daughter, Merab. I want to give her to you in marriage. Only be a brave warrior for me and fight the battles of the Lord." For Saul thought, "There's no need for me to raise my hand against him. Let it be the hand of the Philistines!"

I pray thee, forgive the trespass of thine handmaid: for the LORD will certainly make my lord a sure house; because my lord fighteth the battles of the LORD, and evil hath not been found in thee all thy days.