The events of this chapter apparently took place before Saul's visit to the witch of En-dor, for Saul was killed the day following that visit (ch.28:19), and the battle in which he was killed did not take place until after David's defeat of the Amalekites recorded in chapter 30:16-20.
In verse 1 the Philistines and Israel are seen preparing for battle. As the armies were marching in their various units, David and his men are seen marching with Achish. This alarms the leaders of the Philistines, who demand, "What are these Hebrews doing here?" (v.3). Achish is fully ready to defend David, telling them that David had been with him for many days, in fact amounting to years, and Achish had found nothing to blame him for. (If Achish had known the truth, he would not have been so confident!)
The princes of the Philistines were understandably angry at the very suggestion of a Jewish unit present when they were fighting against Jews. They absolutely insist that David be not permitted to go with them (v.4). As they say, would this not be an ideal opportunity for David to turn and fight against the Philistines in order to reconcile himself to Saul? They did not forget what was common knowledge that in dancing and celebrating, Israel had sung that Saul had killed his thousands and David his ten thousands (v.5).
Achish had no alternative. Calling David, he told him that though he himself fully approved of David personally and wanted him to accompany him to the battle, nevertheless the Philistine rulers were opposed to this (v.6). Therefore he asks David and his men to leave. David certainly had reason to be most thankful to God for this turn of events, but he did not want to give that impression to Achish. Deceitfully he protested, asking what he had done to disqualify him from going to "fight against the enemies of my lord the king" (v.8). If Achish had known what David had done, he would have had a convincing answer! Notice that David does not expressly speak of fighting against the enemies of Achish, but against those of "my lord the king." Achish of course thought that David meant the former, but he did not know that David still considered Saul to be his lord the king (ch.26:17). But David was in a compromising position from which God graciously gave him a convenient release.
Achish repeats his commendation of David in verse 9, and his word as to the attitude of the princes of the Philistines. Therefore he asks him to leave early in the morning with his men (v.10). David would certainly be greatly relieved as he acceded to this and left to return to Ziklag.