Call of Duty: United Offensive is an expansion pack for the first-person shooter video game Call of Duty. It was developed by Gray Matter Studios, with contributions from Pi Studios, and published by Activision. It was released for Microsoft Windows on September 14, 2004, and for Mac OS X on November 22, 2004.

Call of Duty is a 2003 first-person shooter game developed by Infinity Ward and published by Activision. It is the first installment in the Call of Duty franchise, released on October 29, 2003, for Microsoft Windows. The game simulates infantry and combined arms warfare of World War II using a modified version of the id Tech 3 engine. 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 campaigns of World War II in Europe.