What is the expansion pack for the original Call of Duty called?

The expansion pack for the original Call of Duty is Call of Duty: United Offensive.

Released in 2004, United Offensive was developed by Gray Matter Studios, with contributions from Pi Studios, and published by Activision. It expands the 2003 World War II first-person shooter Call of Duty rather than serving as a separate numbered sequel. Its campaigns follow American, British, and Soviet soldiers through battles including Bastogne, Sicily, and Kursk.

The expansion was especially notable for enlarging multiplayer. It introduced bigger maps, vehicles such as tanks and jeeps, a ranking system, and modes including Domination, Capture the Flag, and Base Assault. Those additions helped distinguish it from the original game's more straightforward multiplayer structure.

A common mix-up is confusing United Offensive with Call of Duty 2, which was a full sequel released in 2005. Roads to Victory was a later PSP handheld game, Force Recon was a mobile Modern Warfare 2 spin-off, and Final Fronts was a PlayStation 2 adaptation associated with World at War.