Activision today announced that Call of Duty: United Offensive, the first expansion pack to the military action title developed by Infinity Ward, has gone gold. United Offensive, developed by Gray Matter, contains 13 new single-player missions, 11 new multiplayer maps, and new weapons, abilities, and special effects. Three new multiplayer modes round out the package. United Offensive is slated to ship on September 14 and will retail for $29.99.