An expansion, called Combat Mission: Shock Force - Marines was released on 26 September 2008 and marketed by direct sale through Battlefront's website, requiring Combat Mission: Shock Force to play. The module was sold by direct download, physical disc, or both, and included the latest patch for the Shock Force base game, which was also made available via free download to all non-Marines customers and works without the module for owners of the base game. Content of Marines include an increased number of units, 15 standalone scenarios, an 18-mission campaign, and 25 new Quick Battle maps. New units represent United States Marine Corps and Syrian Airborne troops, as well as regular Syrian units such as the T-90 tank.
Source: Wikipedia, "Combat Mission: Shock Force", available under the CC-BY-SA License.