1 Samuel 28:1

And it came to pass in those days, while David was still living in their country, that the Philistines gathered their armies together for warfare to fight with Israel. It was a mustering of all the soldiers of the Philistine city-states that were fit for military duty; for the purpose was not a mere raid, but a campaign of conquest. And Achish said unto David, Know thou assuredly, since the circumstances were such as David saw before his eyes, that thou shalt go out with me to battle, thou and thy men, as a part of the Philistine army. This was the predicament, the danger, into which David had brought himself by his flight into the land of the Philistines.