1 Samuel 29:3
Then said the princes of the Philistines, What do these Hebrews here? And Achish said unto the princes of the Philistines, Is not this David, the servant of Saul the king of Israel, which hath been with me these days, or these years, and I have found no fault in him since he fell unto me unto this day?
David's crisis came when he was mustered to march with the Philistine forces against Israel, a place where loyalty to his own people forbade him to take up arms against them. At that moment Achish stood up for him before the Philistine princes, presenting him as Saul king of Israel's servant who had dwelt with him a long time without giving cause for blame. The lords nevertheless distrusted him precisely because he was an Israelite, fearing he would turn on them in the midst of battle, and Achish's commendation could not move them to keep him with the army.
To leave the field exposed him to the charge of cowardice, ingratitude, and treachery toward Achish, who had sheltered him and placed confidence in him. To fight would have branded him a traitor to his country and to God's people, endangered his way to Israel's throne, and fixed blame on him if Saul fell in the engagement.