War Mode is the only way to participate in open-world PvP. You can opt-in from your talent pane while in your faction's major capital city—Stormwind for the Alliance, and Orgrimmar for the Horde.

While in War Mode, you receive a 10% bonus to experience and gold, but can only interact with other War Mode players. To leave War Mode, return to your capital and deselect the War Mode icon in your talent pane.