The story behind the coin
The United States Army is older than the United States. The Continental Congress voted the Continental Army into being on June 14, 1775 — more than a year before the Declaration of Independence. So when Congress decided the Army deserved its own coin, it was honoring an institution that predates the flag, the Constitution, and the dollar itself.
That decision became law on December 1, 2008, when President George W. Bush signed the United States Army Commemorative Coin Act (Public Law 110-450). It authorized a three-coin set for 2011: a $5 gold piece, a half dollar in ordinary clad metal, and this — a 90% silver dollar.
The coins had a job beyond celebration. A surcharge built into every sale — $10 on each silver dollar — was sent to the Army Historical Foundation to help build the National Museum of the United States Army at Fort Belvoir, Virginia. In other words, collectors who bought this coin were quietly paying to give the Army its first national museum. (A commemorative coin doesn't circulate; it's sold by the Mint to collectors at a premium, and here that premium did real work.)