Here is the list of Multiplayer Ranks you can unlock in Halo Wars for the Xbox 360, and how many points you need to unlock them.
Recruit – Play one game on Xbox Live
Lieutenant – 80,000 points
Captain – 200,000 points
Major – 400,000 points
Commander – 800,000 points
Colonel – 1,600,000 points
Brigadier – 2,400,000 points
General – 3,200,000 points