ARMA 2: Operation Arrowhead (often abbreviated to ARMA 2: OA) is a stand-alone expansion pack of the tactical shooter video game developed by Bohemia Interactive, ARMA 2. The game includes three new maps, new vehicles, new factions and a new campaign. On August 29, 2010 a downloadable content pack named British Armed Forces was released worldwide which added the British Army faction to the game, including units and new vehicles.
ARMA II: Operation Arrowhead is much like its predecessor in the way it allows users to command units like a real-time strategy game, it also uses a customized version of Real Virtuality 3 (build 72888). Players are allowed to perform duties that real soldiers would perform during combat. In addition to campaigns, players are allowed to play mini-missions and participate in a simulated training program.
The game takes place 3 years after the Chernarus conflict depicted in ARMA 2 and is set in the fictitious country of Takistan. American forces are sent to Takistan to restore peace to the country. Players have the ability to perform optional tasks through out the game, allowing for multiple endings.
Source: Wikipedia, "ARMA 2: Operation Arrowhead", available under the CC-BY-SA License.