2 Samuel 15:14 - Matthew Poole's English Annotations on the Holy Bible

Arise, and let us flee; for though the fort of Zion was strong and impregnable, and he might have defended himself there; yet he had not laid in provisions for a long siege; and, if he had been once besieged there, Absalom would have got speedy and quiet possession of his whole kingdom; whereas if he marched abroad, he might raise a considerable army for his defence, and the suppression of the rebels. Besides, the greatest part of Jerusalem could not be well defended against him. And he suspected that a great number of the citizens might take part with Absalom, and possibly deliver him up into Absalom's hands. Besides, if he had made that the seat of the war, he feared the destruction of that city, which he vehemently desired to preserve, because it was the chief and royal city, and the place in which God had appointed to put his name and worship. Moreover, when David considered that God's hand was now against him, and that he was now bringing evil upon him out of his own house, as he had threatened, 2 Samuel 12:11, it is no wonder if he was intimidated and disposed to flee.