Call of Duty: United Offensive served as the expansion pack of the very successful first-person shooter PC game Call of Duty. Developer Gray Matter Interactive released United Offensive in 2004, replacing the creators of the original game, Infinity Ward.
Much like the initial Call of Duty, players assume the roles of American, British, and Russian soldiers fighting on different fronts during World War II. The single-player campaign consists of 13 missions spread among the three different soldiers. These missions vary from infantry-based combat to vehicular-based fighting, such as manning the guns of a B-17 bomber and commanding a Russian tank. Players must complete several objectives in sequential order to finish a mission. The gameplay and overall single-player experience of United Offensive remained relatively unchanged from Call of Duty. Players still regenerate lost health by using health packs scattered throughout the missions, and the game added only a handful of new weapons, such as the Browning .30 Caliber and M1A1 Bazooka.
Most changes in United Offensive came in the game's online multiplayer. Featuring both free-for-all and team-based action, a wide variety of game modes pitted player against player. Notably, United Offensive introduced vehicular combat into multiplayer, a first for the Call of Duty series. The game also saw the introduction of a ranking system. As players progressed through a match, they earned points for actions that benefitted their team. Players achieved promotions to higher ranks when they earned enough points, and also received bonuses such as extra ammunition, all the way up to a pair of binoculars which could be used to call in artillery strikes. Significantly, the ranking system introduced in United Offensive continued to be improved and expanded upon, and now proves to be an icon of what makes Call of Duty's multiplayer a juggernaut among first-person shooters.