The story behind the coin
The United States Army was born on June 14, 1775 — older than the country it serves. By the time of its 236th year, Congress decided that birthday deserved a coin.
The vehicle was the United States Army Commemorative Coin Act of 2008 (Public Law 110‑450), which ordered up three coins for 2011: a $5 gold piece, a silver dollar, and this clad half dollar. Clad just means a sandwich of metals — a copper core faced with copper‑nickel — the same everyday recipe as the Kennedy half dollar in your pocket. Commemoratives like this aren't spent at the store. The Mint sells them straight to collectors, at a premium, with part of the price set aside for a cause.
Here the cause was a building. Every half dollar carried a $5 surcharge earmarked for the Army Historical Foundation, to help fund the National Museum of the United States Army at Fort Belvoir, Virginia. Buy the coin, and you put a few dollars toward a museum that wouldn't open its doors until 2020.