How to make an enemy attack you when he is attacked by you ????

Hi guys I am making a mmorpg and i want a simple or hard enemy ai both in javascript or c#.So i would like the script to work like this : when you have an enemy in front of you he doesnt attack you but when you hit him once i want to make him chase you for some range and attack you .Hopefully we can get to a script that works :D.