Buy the game on GOG.com (where the key is managed for you) or purchase a sealed physical copy on eBay. If you must use a Steam key, generate it through the Steam client and be prepared to install the 1.51 community patch.

XXXX-XXXXXX-XXXXX (Often started with letters like XE or CS depending on the region).

Call of Duty: United Offensive is a masterpiece. It represents a time when expansions added massive value—doubling the weapon count, introducing vehicles, and increasing the chaos to 64-player battles. Protecting its legacy starts with using a legitimate CD key to keep the tiny, dedicated multiplayer community alive.