Charles Spurgeon Commentary 2 Samuel 15:31

"And one told David, saying, Ahithophel is among the conspirators with Absalom. And David said, O Jehovah, I pray thee, turn the counsel of Ahithophel into foolishness." — 2 Samuel 15:31 (ASV)

Ahithophel was David's choicest friend, companion, and counsellor, yet he had failed him in his time of need. David could use the weapon of all-prayer when he could use no other, and this is like the flaming sword at Eden's gate which turned every way. It will slay our foes if they come from hell; it will drive away Satanic suggestions; it will overcome our adversaries if they come from earth; it will sanctify our afflictions even if they come from heaven.

To know how to pray is to know how to conquer. David checkmated Ahithophel when he said, O Lord, I pray thee, turn the counsel of Ahithophel into foolishness.