On the Connections page of your Battle settings, look for Steam in the Connected Accounts section and click Connect. Blizzard will then send you to the Steam website. Enter your Steam credentials if you aren't already logged in, then click sign in.
Call of Duty on Steam. Welcome to Call of Duty HQ, the home of Call of Duty: Modern Warfare III, Call of Duty: Modern Warfare II and Warzone!".
