In September 2004, an expansion pack called Call of Duty: United Offensive, which was produced by Activision and developed by Gray Matter Studios and Pi Studios, was released. The game received critical acclaim and won several Game of the Year awards from reviewers. Much of Infinity Ward's development team consisted of members who helped develop Medal of Honor: Allied Assault.

This led to a greater emphasis on squad-based play as opposed to the 'lone wolf' approach often portrayed in earlier first-person shooter games. The game introduced a new take on AI-controlled allies who support the player during missions and react to situational changes during gameplay. Much of its theme and gameplay is similar to the Medal of Honor series however, Call of Duty showcases multiple viewpoints staged in the American, British, and Soviet theaters of World War II. The game simulates infantry and combined arms warfare of World War II using a modified version of the id Tech 3 engine. It is the first installment in the Call of Duty franchise, released on October 29, 2003, for Microsoft Windows. Call of Duty is a 2003 first-person shooter game developed by Infinity Ward and published by Activision.

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