Charles Ellicott Commentary 2 Samuel 17:1

"Moreover Ahithophel said unto Absalom, Let me now choose out twelve thousand men, and I will arise and pursue after David this night:" — 2 Samuel 17:1 (ASV)

Pursue after David this night. — Ahithophel saw clearly that Absalom’s success depended on striking an immediate blow. He felt confident, and perhaps with reason, that David in his distress and weariness was in no condition to resist a sudden onset. That his counsel was wise is made evident by the opposition of Hushai and the anxiety to send word to David with all haste. “This night” is generally understood to mean the night of the day on which David left Jerusalem; but from 2 Samuel 17:16 and 2 Samuel 15:28, it appears that he was already encamped by the fords of the Jordan, a greater distance than he could have accomplished in one day’s march.