Charles Ellicott Commentary 1 Samuel 18:17

"And Saul said to David, Behold, my elder daughter Merab, her will I give thee to wife: only be thou valiant for me, and fight Jehovah`s battles. For Saul said, Let not my hand be upon him, but let the hand of the Philistines be upon him." — 1 Samuel 18:17 (ASV)

Behold my elder daughter Merab, her will I give you to wife. —This was only the fulfillment of a much earlier promise. The king had said he would give his daughter in marriage to the hero who slew the Philistine giant champion. For one reason or another he had declined, or at least postponed, carrying out his pledge; and the dark thought crossed his mind, Could he not endanger the hated life, while appearing to want to keep the old promise? He speaks of the Philistine war as the Lord’s battles.

This was a feeling which inspired every patriotic Israelite. “He was,” when fighting with the idolatrous nations, “warring for the Lord”—so David felt when he spoke of the Philistine giant as having defied the ranks of the living God, and alluded to the battle as the Lord’s (1 Samuel 17:26; 1 Samuel 17:47). The same idea is expressed in the title of that most ancient collection of songs which has not been preserved to us—Book of the Wars of the Lord (Numbers 21:14).