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1 Samuel 29:8 And David said unto Achish, But what have I done? and what hast thou found in thy servant so long as I have been with thee unto this day, that I may not go fight against the enemies of my lord the king? 9 And Achish answered and said to David, I know that thou art good in my sight, as an angel of God: notwithstanding the princes of the Philistines have said, He shall not go up with us to the battle. 10 Wherefore now rise up early in the morning with thy master’s servants that are come with thee: and as soon as ye be up early in the morning, and have light, depart. 11 So David and his men rose up early to depart in the morning, to return into the land of the Philistines. And the Philistines went up to Jezreel.
I believe that it is inconceivable that David would have fought against Israel on the side of the Philistines.
When he uses the phrase ‘my lord the king’ David is referring to Saul who was God’s anointed king, but knows that Achish will think that he is referring to him (Achish).
We see a similar ambiguity in ‘thou shalt know what thy servant can do’ when Achish tells him he is going with him to battle.
1 Samuel 29:1 And it came to pass in those days, that the Philistines gathered their armies together for warfare, to fight with Israel. And Achish said unto David, Know thou assuredly, that thou shalt go out with me to battle, thou and thy men. 2 And David said to Achish, Surely thou shalt know what thy servant can do.
I believe that the Lords of the Philistines were correct in their assessment of what David would do.
1 Samuel 29:4... the princes of the Philistines said unto him, Make this fellow return, that he may go again to his place which thou hast appointed him, and let him not go down with us to battle, lest in the battle he be an adversary to us: for wherewith should he reconcile himself unto his master? should it not be with the heads of these men?
God then kept David out of the battle by the incursion of the Amalekites. David had no option but to follow after them.
I hope you have found this helpful.
May God bless you,