A lie?

So David and the six hundred men with him left and went over to Achish son of Maoch king of Gath. David and his men settled in Gath with Achish. Each man had his family with him, and David had his two wives: Ahinoam of Jezreel and Abigail of Carmel, the widow of Nabal. When Saul was told that David had fled to Gath, he no longer searched for him.
Then David said to Achish, "If I have found favor in your eyes, let a place be assigned to me in one of the country towns, that I may live there. Why should your servant live in the royal city with you?"
So on that day Achish gave him Ziklag, and it has belonged to the kings of Judah ever since. David lived in Philistine territory a year and four months. 1 samuel 27

In those days the Philistines gathered their forces to fight against Israel. Achish said to David, "You must understand that you and your men will accompany me in the army."
David said, "Then you will see for yourself what your servant can do." Achish replied, "Very well, I will make you my bodyguard for life." 1 Samuel 28

Later on David destroyed the philistines. Did David lie when he said that he would fight by the side of the philistines? Was it wrong to live in the enemy lands even if it was for self defence?
I think that it is just like a thief breaking into your home, you have permission to defend yourself and your family in any way you can. You might take a shotgun to him or hit him with a metal pole, but I don't believe that it is wrong after he has broken into your house. It was the same with David except that the philistines were going to break down Davids people. so He stopped them in a violent way. Also a good example would be when Rahab hid the two spy's that Joshua sent out and said that they were not in her house. Later God blessed her.It is the same with David.